6 replies to this topic

[lqminh]

Hi everybody,

I am studying about NRTL model for non-ideal mixtures. And I have a problem to understand about binary parameters.
Is there anyone have experience in setting up binary parameters for non-ideal mixture using NRTL model?
Please share your idea and some tutorials for setting up binary parameters from empirical equation as well as experiments.
Thanks a lot.

[MrShorty]

It's a very open ended question, which someone could write a book to answer. Can you be more specific about the problem/question you are having?

[lqminh]

As you know, when you simulate some specific process, such as molten salt extraction distillation process, you have to find the binary parameters for putting into vacancies in Aspen database properties.
My problem is that I have to do experiments to find VLE data and then calculate some binary parameter to fulfill these vacancies in Aspen DB. Also, there are many proposed equations for calculate these parameter from NTRL model. However, I do not how to use these equations for calculate them exactly. If you have some guidelines, please share with me. Thanks

[MrShorty]

Also, there are many proposed equations for calculate these parameter from NTRL model. However, I do not how to use these equations for calculate them exactly.

I'm not sure what equations you are referring to. However, one has to be cautious using "estimating" equations for NRTL (or other equation) parameters. If you would post which equation you are referring to here, then we could discuss how to use that equation.

My problem is that I have to do experiments to find VLE data and then calculate some binary parameter to fulfill these vacancies in Aspen DB.

I'm not sure if your question is about performing VLE measurements, or regressing experimental VLE data, or both.

There have been a lot of techniques used for obtaining VLE data. If you are worried about how to obtain the experimental data, I would suggest you search through the literature to find and study different techniques and see which techniques seem appropriate for the specific system(s) you are interested in.

As for regressing data, there have also been different techniques suggested. The basic idea is to select two "givens" from the measured VLE data (usually consisting of pressure (P), temperature (T), liquid (x) and vapor (y) compositions), then calculate the other 2 variables using the desired VLE model (in your case you seem to want to use a gamma-phi model with the NRTL equation representing the activity coefficient).

Perhaps the simplest equation for this is to use a "modified" Raoult's law: P(i)=gamma(i)*x(i)*P0(i) where P(i) is the partial pressure of i, gamma(i) is the activity coefficient of i, x(i) is the liquid composition of i, and P0(i) is the vapor pressure of pure i. Using this equation at a given T and x, it is relatively easy to calculate P and y. Then you can adjust the parameters of the activity coefficient equation to obtain a "best fit" of the VLE data.

[breizh]

Hi ,
This resource may help:

http://www.engg.uaeu...r/downloads.htm

Breizh

Edited by breizh, 08 September 2011 - 12:11 PM.

[lqminh]

thanks all

[siretb]

It is not that difficult.
First you have to understand exactly what the NRTL model is. Google NRTL , and you will find good introductions (Wikipedia is not bad).
for many applications ideal vapor phase is just OK.
Then, you have to perform VLE experiments at two different pressures (one is not enough, except for special cases), using binary mixtures.
And just run a regression program. Some simulators (ASPEN) have one built in, and it works well.
The difficult point is to have reliable x-y-T-P data.
enthalpy (excess enthalpy) dat can also be used.
if you have little data, setting alpha to its default value of 0.3 in the NRTL can be fine (not always)