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K Values For Relative Volatility Calculations


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#1 joshpeacock

joshpeacock

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Posted 16 February 2018 - 11:00 AM

I am designing a multicomponent distillation column to separate oxgygen and nitrogen (mainly) and i am trying to calculate the relative volatilities for the top and bottom. Does anyone know a source that would enable me to find K values for any substance at any pressure/temperature?



#2 MrShorty

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Posted 16 February 2018 - 02:14 PM

Any substance at any pressure/temperature? That is a pretty tall order. I am not aware of any single source that would yield K values for any substance at any pressure/temperature.

The usual approach to this question is to:

1) Gather experimental data. There are databases out there, like the DeChema and Dortmund Data Banks that have compiled a substantial chunk of VLE data from the open literature and some private communications. Sometimes they include interaction parameters for common thermo/VLE models.

2) Select a suitable thermodynamic model and derive suitable "interaction parameters" for the model from the experimental data.

3) Use the thermo model to compute K values at appropriate temperatures and pressures.

 

If you have access to a good simulator package and you trust it (meaning you have checked it against something that you know is right), they have usually done steps one and two for you, and can readily perform step 3. For common "air" components, I would expect a good simulator to be able to do this fairly well (always with a healthy mistrust of what comes out of the computer's "black box"). For common "air" compounds, I would not be surprised if there are publications by groups like the GPA that may also have suggestions and nomographs for K values of common air components at typical distillation conditions for those components.

 

There should be good sources out there for air components. It seems that there are just too many holes in our databases, though, to assume that simulators and other publications will allow us to find K values for all compounds at all temperatures and pressures.






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