3 replies to this topic

[babucher]

Greetings to everyone,

My question concerns the vapor composition of a multicomponent mixture evaporating (at 1 bar) from a sub-boiling-point liquid. That is, if I had a 50/50 (by mass) mixture of water and ethanol at 50 C, what would the vapor composition look like, as the liquid begins to evaporate?

Options:
1. Whether boiling or not, still use the VLE curves drawn on the Txy diagram at 1 bar.
2. The correct data to use would be the VLE curves drawn on a Txy diagram at the pressure at which the 50/50 mixture _would_ boil.
3. Other (depends on the partial pressures of the components in the atmosphere, depends on non-ideality factors which are generally taken into account using models like NRTL and UNIQUAC, etc...)

What are your thoughts?

Brian

[MrShorty]

I'm not quite certain I understand all of the details, but here are a couple of thoughts. First thing to note, I am assuming the system is a 50/50 mix of ethanol/water at 50 C and 1 bar. You are asking what the vapor looks like just as the liquid begins to evaporate.

Scenario 1: System is contained in a thermostatted floating piston type container. At 50 C and 1 bar, we are "below" the bubble point of the mixture, so there will be no vapor phase to speak of. Vaporization won't occur until we either heat the mixture to the bubble point for a 50/50 mix at 1 bar or decrease the pressure until we reach the bubble point for a 50/50 mix at 50 C.

Scenario 2: Pressure is maintained by a nitrogen blanket. In this case, the partial pressures of the ethanol and water in the vapor phase will be determined by VLE calculations (activity coefficient equation should be fine) done at 50 C rather than at 1 bar. I think this is essentially what you are proposing with option 2.

Hope that helps.

[babucher]

I'm not quite certain I understand all of the details, but here are a couple of thoughts. First thing to note, I am assuming the system is a 50/50 mix of ethanol/water at 50 C and 1 bar. You are asking what the vapor looks like just as the liquid begins to evaporate.

Scenario 1: System is contained in a thermostatted floating piston type container. At 50 C and 1 bar, we are "below" the bubble point of the mixture, so there will be no vapor phase to speak of. Vaporization won't occur until we either heat the mixture to the bubble point for a 50/50 mix at 1 bar or decrease the pressure until we reach the bubble point for a 50/50 mix at 50 C.

Scenario 2: Pressure is maintained by a nitrogen blanket. In this case, the partial pressures of the ethanol and water in the vapor phase will be determined by VLE calculations (activity coefficient equation should be fine) done at 50 C rather than at 1 bar. I think this is essentially what you are proposing with option 2.

Hope that helps.

MrShorty,

Thanks for the reply. Yes I am curious about scenario 2.

Let me make sure I understand. My Option 2 would be the one to apply for your Scenario 2. That is, I would find/generate a set of VLE curves at the pressure that corresponds to boiling the 50/50 (by mass) ethanol/water at 50 C. I would then use those VLE curves, even though my system isn't actually at the pressure the VLE curves were generated at.

Did I get you right?

If so, do you have any references where this is discussed?

Thanks,
Brian

[MrShorty]

I think you've kind of got the idea, only, instead of generating Txy curves at whatever P you decide is appropriate, generate Pxy curves at a fixed temperature of 50 C. I know sometimes we get used to thinking of these things in terms of isobars, but, in this case, I think it will make more sense to think of it as an isotherm.

I suppose if you wanted to deal with this rigorously, you would treat it as a ternary system H2O/EtOH/N2, calculate the solubility of N2, and then use that in your calculations. But the solubility of N2 will be small, and the effect of that small amount of dissolved N2 should be negligible, so we are essentially pretending it's not there.

In essence, what we're doing is a constant volume flash calculation at 50 C and whatever volume your vessel is.