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Thermodynamic estimation of phases

Thermodynamic estimation of phases

Thermodynamic estimation of phases

Hi Everyone.

I am trying to understand how to determine the phase from the following list of inputs:

total mole fraction of each species (as an example: 10 mol/h NH3 + 5 mol/h Water + 4 mol/h CO2)
temperature (65 degC)
pressure (4 barg)

By doing hand calculations or a python algorithm.

My initial idea is to look at the saturation pressure of each pure component at the specified temperature with clausius clapeyron and evaluate whether the sum is higher than the actual pressure. If yes then the phase must be a liquid. If not then it is either two phase or gas. But how do I determine whether it is two phase or pure gas?

If the components does not behave ideally in the gas or liquid phase adjustments in the form of activity coefficients and fugacities must be used, which complicates matters a lot.

I have yet to find a book that simplifies this for me. It is nearly always assumed you know the initial phase...

I would really appreciate some input either in the form of links to a book or a reply.

Thanks in advance.


RE: Thermodynamic estimation of phases

A simulator, like Aspen Plus, would tell you the phase(s).

Using the Unsymmetric Electrolyte NRTL property model, and the Redlich-Kwong equation of state for vapor phase properties, I quickly (minutes) got from FLASH2 at 65 C and 4 barg:
FEED = 19 mol/hr
VAPOR = 12.8 mol/hr
LIQUID = 6.2 mol/hr

As for books, try your college thermo book(s) and the latest edition of The Properties of Gases and Liquids.

Good Luck,

RE: Thermodynamic estimation of phases

Hi Latexman.

Thanks for the good suggestions and the calculation but I am looking for a link to a way to calculate it by "hand" (in python probably). I have become way to dependent on simulators over the years that I have completely forgotten to do it manually..

I want to rediscover the core concepts and I simply cant get my head around how to determine which phases are present unless I have a phase diagram...

RE: Thermodynamic estimation of phases

There's a lot of open source, "canned" Python programs, subroutines, and functions out on the web. Have you searched for exactly what you want?

Good Luck,

RE: Thermodynamic estimation of phases

Quote (Donmiguel)

But I am still struggling with the theory...

Start at the beginning with simplest/most basic vapor-liquid equilibrium, even if it does not predict the real system very closely, them add enhancements/complexity one step at a time to better match the real system. Keep it in neat, hand-written journal style to document the progress and serve as a record.

My prior advice, "As for books, try your college thermo book(s) and the latest edition of The Properties of Gases and Liquids" should help with the theory.

Also, Google a simple description of what you are looking for, like "how to calculate vapor-liquid equilibrium". I see some You Tube videos that look promising. Then, enhance the searches as you add enhancements/complexity one step at a time. Document the worthwhile information in the journal by hand so you learn it.

Good Luck,

RE: Thermodynamic estimation of phases

for phase equilibria one can solve fg(1)*x(1)=fg(2)*x(2)=... where fg and x are the component's fugacities and fractions in the different phases (vapor, liquid, solid...)
it shouldn't be too difficult for vapor-liquid assuming for example ideal state laws (i.e. vapor pressure for liquids and operating pressure for vapors), for constant K values there are relatively simple procedures (see Rachford and Rice, Petroleum Trans AIME 1952),
but not easy with the modern, complex, equations of state able to predict the strong composition-dependence of many thermodynamic properties,
simulators and libraries (I utilize Prode Properties which has a interface to Python) include a set of solvers for the different operations (T-P, H-P, H-T, S-P...)

for the theory I would recommend (depending from your level)
1) Phase Equilibria in Chemical Engineering (Walas)
2) Thermodynamic models: Fundamentals & Computational Aspects Michelsen / Mollerup

but there are many others available...

RE: Thermodynamic estimation of phases

NH3 reacts with CO2 in solution, and that is going to mess things up a lot.
There is a thermo model in Pro/II - Simsci meant for acid gases in solution which also includes ammonia - its used for sour water stripper simulations, if I remember correctly. See if you can find narratives on this in the Applications Guidelines Manual or the CaseBook for Simulator Applications.

RE: Thermodynamic estimation of phases

Hi Everyone. Thanks for the many good input. It is highly appreciated! Will try to post how I progress. Br Michael

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