8 replies to this topic

[Mehdi Alizad]

Hi,
Do you know what is the best equation of state to use inside AspenPlus for the following gas stream:

Gas stream containing hydrocarbons(C1 mostly) with sour gases (H2S and CO2,...) and water
at high pressure (67bara)
and temperature about about 40C

Please tell me the reasons for choosing suitable EOS.

Thanx

Edited by Mehdi Alizad, 30 August 2012 - 06:32 AM.

[Mark-TR]

Hi,

For most hydrocarbon applications; where no pseudo-components are considered;t he most used method is peng-robinson, other common method used is redlich-kwon-soave . Please note that a typical problem while modeling hydrocarbon with water is that the water with hydrocarbon generates V-L-L equilibrium.

Contrary to other softwares; such as HYSYS, Aspen does not consider free water by default. If free water is not selected water is modeled with hydrocarbon correlations. Consequently, strange results may be observed. Therefore, don’t forget to activate the free water method at set up/specification in your model.

I have had several discussions with Package Vendor of purification units; such as PSA; because of this common mistake.

Attached you will find a document regarding the most commonly asked question to ASPEN where a detailed discussion of how to select the proper free water method is provided.

ASPEN software also has an assistant that can help you to select the proper method for your process simulation.

In any case, I have to clarify that all EOS are only an approach to reality and may be more or less accurate according to process conditions simulations and composition. The only way to ensure that the model is correct is to compare it to real data.

kind regards

Attached Files

•  60_Commonly_Asked_Petroleum_Application_Questions_About_Aspen_Plus1.pdf   98.57KB   53 downloads

Edited by Mark-TR, 30 August 2012 - 08:02 AM.

[Mehdi Alizad]

Thank you for providin info Mark-TR
as u mentioned it is better to choice PR or SRK equation of states for high pressure light hydrocarbons.
but is there any suggestion to choice between them? i mean any instruction, help, guide? clearly which one has results closer to real?
I'm looking for any reason to choice between them.

The Gas composition:
84% C1
5% C2
0.1% water
0.69% H2S
1.82% CO2
3.5% N2
+ other hydrocarbons

Thanks again

[PaoloPemi]

I understand that one has to accept the limits of the software available but perhaps you may try some different feature to improve the accuracy of simulation,
a stream including 0.1 % of water , 0.69% H2S , 1.82% CO2 can be difficult to model with SRK or PR and standard (VDW) mixing rules, you may set a different model as CPA or a EOS with mixing rules as Huron Vidal or Wong Sandler, in none of these are available PSRK could also be a reasonable alternative.
I utilize a different software (Prode Properties, see prode.com) which includes specific models for hydrocarbons + water and polar fluids, however I suppose that Aspen can do the work once the thermo package has been properly tuned.

[Technical Bard]

The PR equation was originally built based using methane and the regression of parameters fits best to methane. SRK was originally fit to hexane. However, tuning of the parameters over the years by the simulator developers means that either works well. Indeed, many simulators use 'improved' versions. Pro/II's SRK model is "improved", whereas HYSYS has it's own PR version, and VMGSim as something called "Advanced PR"

[pter]

Bard,
I think there is a little mistake,
according several books and papers original SRK produces the best results for fluids with low acentric factor (say < 0.3) while PR for fluids with higher acentric factor,
of course regressing parameters can improve accuracy.
Also for water + hydrocarbons I would suggest to consider specific models (GERG, CPA etc.)

[Technical Bard]

pter -if you generate a bunch of properties for light gas mixtures with PR and SRK in any simulator today, you will find there is essentially no difference in the results. This is because the regressed data has tuned them to match actual data. If you designed a natural gas plant with SRK or PR I wouldn't complain about it. Now if you get down to some specialized bits (C2 splitters) things get a little messy, but that isn't a "typical" scenario.

[Mark-TR]

Dear all,

Just enrich a little more this topic, I found in my reference documentation some guidelines on selecting thermodynamic models from the university of Columbia. I´d like to highlight that another important point while selecting a thermodynamic model is the process related to the simulation, are you modeling VLE, LLE or just hydraulics? This is discussed in the attached document.

As stated by Technical Bard, selecting between PR or SRK should not be a problem.

Kind regards

Attached Files

•  Selecting thermodynamic models.pdf   29.21KB   71 downloads

Edited by Mark-TR, 03 September 2012 - 10:31 AM.

[Mehdi Alizad]

Dear all

firstly thank u all guys for helping

Mark-TR, it is about modeling VLE

actually i found no significant difference between PR and SRK, except their limitation of pressure and temp usage

I decided to use SRK.