15 replies to this topic

[daraj]

Hi, often times, a lot of us go with thumb rules for selecting thermo package in siumulators for conceptual designs/flowsheeting purposes. Like Peng -Robinson/SRK for hydrocarbons, if it is polar compounds dominated then choose NRTL-RK and so on.

 

I would like to know the basis, from first principles, behind the choice of VLE for such applications. For nstance why is Peng-Robinson suitable for a lot of petrochemical/hydrocarbon processes, why d we switch to NRTL or something else when polar compounds are involved and so on. Are there any materials/resources/threads here available where the scientific basis for such choices are explained?

 

Most of the time flowcharts are given to users, to choose a thermo package based on certain default questions and answers about the systems they are rying to model. Iam trying to understand in more depth the reasons behind these choices instead of blindly following the guidelines.

Your inputs are greatly appreciated

[shvet]

Hi

Look for:

- Shell Design and Engineering Practice. Basic Data and Phase Behaviour Methods. Manual. DEP 20.00.10.10 (I have version of 2003 year)

 

If you want more theory than:

- Robert C.Reid, John M. Prausnitz, Thomas K. Sherwood The properties of Gases and Liquid (I have 3d ed.)

- see referiences in DEP 20.00.10.10

 

If you cant find them in google - write me I share it.

 

As for me and my colleagues the best proof of thermo package suitability is a real process. We do in this way:

- find the same process

- make model

- change process parameters

- check model

- apply model to new process

[chrbac]

I would try an Aspen user guide.  

This is one I use for physical property methods

http://oce.eng.usf.edu/ManualAspen.pdf

 

Catherine

[abhi_agrawa]

I would recommend this excellent paper - "Don't gamble with physical properties for simulations" by Eric Carlson.

 

http://user.engineer...ble Article.pdf

[breizh]

Hi ,

Consider the document attached , in particular the last slide , download the link to get detailed information.

Good luck.

 

Breizh

[serra]

As general introduction I would recommend the (already mentioned) Properties of Gases and Liquids (Poling, Prausnitz, O'Connel) or similar books,
most documents mentioned in previous posts discuss the features available in a specific software (Aspen) and do not consider recent models which includes association or some complex mixing rules...

also the models may be quite different,
for example the extended Peng Robinson available in Aspen is very different from the extended Peng Robinson available in Prode (which has 5 parameters per chemical allowing a more accurate reproduction of vapor pressure and liquid phase...).

Of course the availability of some specific model or feature depends from your software, however you can try (within some limits) to adapt a general purpose model providing you have access to a utility allowing to regress data.

In my experience, recent EOS with temperature dependent parameters allow to manage even polar fluids (say water)
as example the extended Peng Robinson available in Prode allows to predict water dew point in hydrocarbons (up to 100 Bar)  with an accuracy of about 1-1.5 K ,

I have adopted the same model for water-methanol and similar mixtures with good results.

For very non-ideal, polar, mixtures at high pressures you can adopt a EOS such as Extended Peng Robinson + NRTL with complex mixing rules (say, for example WS or MHV2)

Other models such as CPA or SAFT can accurately reproduce vapor pressure allowing good results for phase equilibria of many (including associating) fluids...

Finally, many models such as Pitzer, E-NRTL, E-UNIQUAC, CPA-Electrolyte  etc. are available for electrolytes, the oldest (Pitzer) is probably the most flexible but requires many parameters, the others have different features depending from the specific variant avauilable in your software....

 

As you see there are no simple rules of thumb and the selection of the thermo package may require evaluation of predicted results vs. experimental data (when available)  

Edited by serra, 20 October 2015 - 03:39 AM.

[daraj]

Thanks to everybody for your valuable inputs!

[daraj]

serra and others, more than choice of models, what Iam interested in understanding is why some models are suited only for certain systems/conditions and not for others? This sort of understanding/insight will help me to choose models better than going by just published guidlines/thumbrules or by trial-and-error. Often in business environment, we might not have time to choose by trial and error.

so, for example if we know that an EOS, say, has been developed using certain assumptions which are not valid beyond a system pressure of 10 bar, then right away you know you should not consider it for high pressure systems. I do not need to consult a flowchart or aspen published guidlines and can rather use my own judgement. That's the insight I want.

Any review articles/peer-reviewed publications or evena  good internat article that summarizes such assumptions/insights would be helpful besides what people have posted above

 

some questions like:

for instance, what were the limitations of earlier Peng-robinson models(in terms of assumptions) and how were they addressed to predict VL equilibrium even for polar fluids in recent times?

 

or under what circumstances using a  Wilson activity model is better than using Peng-robsinson for both vapor and liquid phases? why is it so?

Edited by daraj, 21 October 2015 - 01:13 AM.

[gegio1960]

Most part of the annual course "Chimica Fisica 1", during the 3rd of 5 years studies for the "Chemical Engineering" degree at Politecnico di Milano was dedicated to this matter.

It was a mandatory course, 33 years ago... but I suppose that something similar is also available nowadays in other universities.

At that time the book mentioned before by Serra was the famous "Reid-Prausnitz-Sherwood".

Both Prausnitz and Sherwood have given their names to two adimensional factors used in chemical engineerig - transport phenomena calculations.

good luck!

Edited by gegio1960, 22 October 2015 - 04:35 AM.

[abhi_agrawa]

Daraj,

 

In order to understand why some models are good for some type of systems and are no good for others, you have to understand the basis of development of those models. For example, the Equations of State were mostly developed to model vapor phase non-ideality, whereas activity coefficient models were developed to model liquid phase non-ideality. Therefore, an EOS would not be a good model for a system that exhibits strong liquid phase non-ideality.

 

If you really want to understand what has gone into the development of the thermodynamic models, I would strongly recommend reading the highly recommended book - Robert C.Reid, John M. Prausnitz, Thomas K. Sherwood The properties of Gases and Liquid. This topic is to extensive to be discussed in general in a forum like this.

 

abhishek

[daraj]

Thanks gegio and abhi

[daraj]

abhu, is there an updated version of the book you mentioned which incorporates recent findings? I dont mind revisiting what is considered as the pioneering book in this area, but at the same time I also want them to be recent enough so that they would have edited and incorporated  some of the newer findings. As an alternative, any other recent books which are very good in this area would also be a useful reference for me.

[gegio1960]

if you read post #6, by serra, you already have the answer: "Properties of Gases and Liquids (Poling, Prausnitz, O'Connel)"

good luck!

[Dr. Mabuse]

Ladies and Gentlemen:

.... Here is another good one !

http://www.eq.uc.pt/...Hill (2011).pdf


Kind regards from Germany,

Thomas

[Dr. Mabuse]

Dear Members,

At paragraph # 2 of this thread reference is made to Shell DEP 20.00.10.10. I've tried to search the document via google but without any success ! Could anybody post a link to this standard !? Many thanx in advance !

Kind regards from Germany,

Thomas

Edited by Dr. Mabuse, 03 November 2015 - 04:34 AM.

[alic4]

I would recommend this excellent paper - "Don't gamble with physical properties for simulations" by Eric Carlson.

 

http://user.engineer...t Gamble Article

this article is very useful, to emphsis I recommend this article