Reading enthalpy chart for liquid and vapor fractions

[Mrepp]

This has been bugging me for a while and since I just joined the Forum, I thought I would post a question. How do you determine the fraction of liquid and vapor from a pressure enthalpy chart when inside the two phase region?

 

[Montemayor]

Mrepp:

The way you use the Mollier (or T-S) Diagram to determine the fraction of liquid and vapor when you have a point inside the "dome" of the diagram is as follows:

You make a material balance around the adiabatic expansion taking place.

F = V + L

F = the Feed
V = the Vapor portion resulting from the expansion
L = the Liquid portion resulting from the expansion

Then you make a heat balance around the adiabatic expansion taking place.

F Hf = V Hv + L Hl

Hf = the enthalpy of the Feed
Hv = the enthalpy of the Vapor formed
Hl = the enthalpy of the Liquid formed

By simple substitution, you should get the vapor fraction:

V/F =(Hf -Hl)/(Hv -Hl)

You can also, of course, get the liquid fraction by similar substition.

Needless to say, you obtain the enthalpy values from the diagram.

I believe this answers your question in a logical and detailed manner.

Art Montemayor
Spring, TX

 

[Mrepp]

It was a graphical approximation, where your point was in the dome..extend a line in both directions and somehow come up with fractions. When you say Mollier most of the charts are P vs. T. I want to use the P vs. Enthalpy because then you can see where the phases are. I got boggled in school with all the charts, psychrometric, TS, TP,

Inside the dome there are no temperatures. What happens to the temperature inside the dome, does it combine/parallel the specific volume lines? I can't remember how to use the chart, what to do if I have temperature and pressure and of a pure substance and I need to use this chart to get the quality.

Let's try say a Butane Pressure vs. Enthalpy chart. If I am at 10 atm for Butane, then sat liquid is ~88 and across to sat vapor is ~215 = 303 total, 215/303*100 = 70.9 % vapor and 29.1% liquid. Is this crap or close to what I am asking?

 

[TD2K]

I think I recall what you are asking Mrepp. I'm going to use my copy of the n-butane Mollier chart in my GPSA as an example.

Let's say you start out with butane at 1000 psia and 180 deg F. If you expand that through a valve to 10 psia, you first draw a point representing butane at 1000 psia and 180F. You then draw a line vertically downwards (as this will be an isenthalphic expansion) to the 10 psia line (line A)

On the LHS is saturated butane liquid. On the RHS is saturated butane vapor.

The % of butane liquid can be estimated by measuring the distance from line A to the RHS (vapor line) divided by the total distance from the saturated butane line to the saturated butane vapor line. And for the %vapor, it's just the reverse.

How to remember which side to take you ask? It's easy. Again, if you look at the horizontal line you have saturated butane vapor on the RHS and saturated butane liquid on the LHS. As you move towards the LHS (eg, towards all butane liquid), the distance between where you are and the RHS is the one increasing (just like you are getting closer to 100% liquid). Similarly, the distance between where you are and the LHS is decreasing, just like the vapor fraction is decreasing. If you think about it that way, you can always figure out which distance measurement correlates with which phase.

The other option is to go back to your vertical line A and read off the mixture enthalphy. From the Mollier chart, you can also read off the enthalphy of the saturated liquid and vapor. Do an enthalphy balance to determine the fraction of vapor and liquid.

 

[TD2K]

One more point, there ARE temperatures in the dome. But, the temperatures lines are horizontal. This is because at a constant pressure (and in this area, a constant temperature), you can have anything from saturated butane vapor to saturated butane liquid.

Thus, if you follow a temperature line through the liquid phase, it goes horizontal once you hit the dome (eg, you starting boiling) and remains horizontal until all the liquid is vaporized when it drops down again.

 

[Mrepp]

Thanks TD2K, and also to Art. I wanted a test run question to try out this new site. Art, I just came out of Houston a year ago from a chem plant out in Alvin - Solutia. I plan on using this resource again and feel a little less alone in my quest for engineering nirvanna. Thanks for helping in remembering that little trick as well as the refresher on the general graphical qualities of the chart was what I was looking for. It is fundamental which was why it was bothering me.