Rules of Thumb for Equations of State 3

[BusyCEO]

Has anyone with experience in applying equations of state (ie. Peng Robinson, Souave Redlich Kwong, BWRS etc.) have any thoughts as to which ones perform best for different ranges of conditions (ie. low pressure, high pressure, light or heavy mixtures etc.)?

thanks
Mack

 

[pmover]

BusyCEO,

slow down....take a big deep breath and let it out slowly...


i trust this helps...

i know that many process engineers use peng-robinson or bwrs in their models, with p-r being the preferred choice, simply because it is a simpler equation. while not thoroughly familiar with hysis or hysim, i do believe there are options available.

from memory, as far as accuracy of results compared with actual data, pr and bwrs provide nearly the same results.

with regards to application of various eos with respect to ranges of pressures, temps, composition, and accuracy of results, i came across a web site (or two) awhile back that provided a concise summary of the results. i do not recall the web site address, otherwise, i would include it in this response (if time allows, i will search again).

i suggest searching the internet using the search criteria of BWRS+equation of state or peng-robinson+equation of state and investigate the results.

my memory is a bit rusty, but i think the data you are after is in a pdf file available on the web.

lastly, i do have some technical papers regarding usage of the bwr and portions of "Fluid thermodynamic properties for light petroleum systems" by ken starling. this documentation does describe some limitations, but it is a limited amount. please advise if desired (do not have all in electronic format!).

-pmover

 

[kbander]

If you are using Hysis, they give some pretty good guidelines as to which EOS or activity model you should be using right in their manual.

 

[gunnarhole]

Dear BusyCEO,

John Edwards at pidesign (in the UK) has written an article about this question and you can see it at the following URL. I think pidesign are Chemcad resellers.

http://www.users.globalnet.co.uk/~jee/Mnl031.pdf

There may be more here than you want to read but there are summary tables included if you want to skip the mathematics.

Regards,

Gunnar

 

[FACostello]

The NIST has many Equations of State at its web site. The people there do much to ensure that the results are at least as accurate as any others.

 

[1305]

The most indicate equation to apply depends on the properties of the fluids you want to study. I recommend you consulting the physical properties of your particular substances. At the documents,(specialy if they are journal articles) is probable you find appointments to the thermodynamic method's of properties and equilibrium calculation.

Using PR is easy to implement a model calculation of equilibrum.

If you only need a general review, they are lots of good books for you, you can find them taking "Equations of State" as key word.

In general trends, for instance, for high pressures and supercritical conditions, PR is appropiate, for liquids wich have desviations of ideality unifac and uniquac are normali the best choose.

 

[hacksaw]

They are all useful as real gas estimates and the all get progressively worse as you approach the liquid state.

R-K is popular because you can generate the constants from the critical data and the accuracy is not too bad.

BWR is much more exact, but is so dependent on some onwe having generated the constants from careful measurments, that you might as well use fluid specific property data that is traceable to measurements.

 

[Galgo]

It is very important that one identify whether or not one is interested in saturated vapor, superheated vapor, or saturated liquid properties. Supercritical properties are a world unto themselves. Mixtures would invite a further complexity.

Peng Robinson is quite applicable to Petroleum industry,
as is the work of John H. Erbar. See Lee-Erbar-Edmister EOS.

It is not clear that you seek a "simple" EOS or an "accurate" EOS as the twain are at conflicting purpose.

Simple is Redlich-Kwong.

Accurate, at the extreme, is Bennedict-Webb-Rubin. Provided one has available all the information to apply it.

I highly suggest that you review "The Properties of Gases and Liquids" by Prausnitz, Reid and Sherwood for excellent guidance in this area with attention paid to the majority of applicable equations of state and mixing rules for multicomponent systems.

For excellent information and assistance, I would suggest that you contact the Mechanical Engineering departements at Oklahoma State University and the University of Oklahoma as they have significant experience in the Petroleum industry and would likely be of assistance.

If you are working near the saturation line, then the following will provide satisfactory information for use in vapor properties up to the critical point:

Pc Vc Mw / (2 J Tc) = 0.272622

P V Mw /(2JT) = 1 - V* Exp ( - V* /3.14 )

V* = (Vc/V)^(2/3)

and will yield accuracies of +/- 2% with the exception of ammonia and water which are highly polar and deviate near the critical point.


Best of luck with your endeavours.

Sincerely,
L. H. Lankford, PE
Fredericksburg, VA.