3 replies to this topic

[Mainak Sarkar]

Please some one help me to understand the vapor liquid equilibrium dynamics.

 

How dose vapor liquid equilibrium (VLE) of a binary component system changes/effected on introduction of a new third component (say non-condensible gas)? For example, say we have a Benzene - Toluene system in a closed vessel at Temperature 60 deg. C and pressure 760 mm HG.  Now the vessel is pressurized by Nitrogen gas (N2). Then, what will be the dynamics of VLE in this situation?

 

Regards,

Mainak.

[MrShorty]

I don't know that there are very many generalizations that can be made for such a broad topic. It should also be acknowledged that this topic can span a significant portion of a thermodynamics of VLE course, and one cannot be expected to cover the topic thoroughly in a single internet forum post.

 

Limiting ourselves to the special case of "high" boiling, semi-volatile compounds with a third, "low" boiling "non-condensible" (these terms in quotes because our classification of something as "high" boiling or "non-condensible" is somewhat arbitrary), I see a few general trends.

 

1) As the amount of non-condensible gas increases, the bubble point generally increases. Or, said another way, as the pressure increases, the solubility of the non-condensible in the liquid phase increases. Quantitatively, this is very different for something like carbon dioxide in an aqueous amine as opposed to N2 in aromatic hydrocarbons.

 

2) As the pressure increases, the impact on the vapor phase fugacity increases. As the solubility in the liquid increases, there is more gas in the liquid to impact the liquid fugacity. So, at "low" pressure (again, "low" vs. "high" pressure is somewhat arbitrary), the impact of the "gas" on the partial pressure of the solvents is small. One would usually not expect much impact (for "inert" compounds like N2 with aromatic hydrocarbons) at "low" pressures. At "high" pressures, this impact can be quite large.

 

3) As the pressure increases, there will come a point where the bubble point and dew point curves meet. This is the "critical point" of the ternary mixture. As the "gas" concentration increases, the VLE of the system will generally trend towards the point where vapor and liquid phases converge.

 

Those are some general trends. You can use your favorite VLE model to attempt to estimate and quantify these effects.

Edited by MrShorty, 08 December 2015 - 12:05 PM.

[Mainak Sarkar]

After lot search I got a equation called after English physicist J. H. Poynting,

 

This equation states that at constant temperature (T) if the pressure (P) of a Vapor-liquid system (at equilibrium) changes by any gas, non-condensable at that T & P and then equilibrium is again established then the vapor pressure increases.

 

The derivation of the equation is given below:  

 

Attached Files

•  J H Poynting.docx   109.19KB   20 downloads

[MrShorty]

Yes, the Poynting factor describes the effect of pressure on the fugacity of a condensed phase. And it is important to include the Poyntiing factor for rigourous, quantitative work (usually when using gamma-phi models), However, the Poynting factor is also usually a minor factor in the overall VLE calculation. I expect that, if you look at the quantities in your standard gamma-phi model, you will find that the effect of pressure on the vapor phase fugacity coefficient is much greater than the effect of pressure due to the Poynting effect. For rigorous work, one would not want to neglect the Poynting correction, but there are certainly much larger factors that effect the VLE calculation.