Phase Equilibrium: Txy Diagram

Organized by textbook: learncheme.com/
A conceptual question investigating how the molar composition changes to a vapor-liquid equilibrium system as temperature is changed. Made by faculty at the University of Colorado Boulder Department of Chemical and Biological Engineering. Check out our Separations/Mass Transfer playlist: • Separations/Mass Transfer
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[@erossi98]

Thank you for such a fantastic video, I appreciate the clarity you have provided in regards to the shape of the graph a great deal- I've never understood it before today.

[@calebanderson1532]

in the middle of a test this one just gave me the answer flat out thanks brother

[@Dawn_1992]

Thank you so much. This actually makes sense now - I have an exam tomorrow and I didn't understand a word of it when my lecturer was explaining, or from the textbook!

[@christianalvarez5409]

Did you pass? are you an engineer now?

[@kimrainman]

Thank you sir. You saved my grade

[@bharathk9594]

What is the vapor composition below the bubble point ? I thought that for mixtures and for pure components there is a strong vapor pressure dependency on the temperature. For example, if I heated a closed system of 0.4 Methanol, 0.6 ethanol from room temperature, I would expect that as the temperature increases more molecules will be in the headspace of the vessel and as such, if I held the temperature at 65 C at 1 bar for days, won't there be an equilibrium between the vapor and the liquid ?

[@apelelekoyo6350]

Thanks for such great videos they are really helpful. A quick question. 1:45 How do you determine how big the Liquid-vapor region?

[@teelit7655]

How would l determine distillate temperature using this diagram?

[@povgfuelgaming7521]

How do you calculate the mole fraction of vapor phase

[@Doctor_Drew]

Hi there. Thank you for your videos. I think I understand the T,x,y diagram for the first problem. However, without going through it, why is it wrong to just think of it conceptually. I would think it's E. If I have a closed beaker with two substances, in this case methanol and ethanol, at some Temperature T and 1 atm pressure, I would THINK that by increasing the temperature, both of the liquids would want to evaporate therefore increasing the vapor phase and decreasing the liquid phase composition. Why is this wrong??

[@Doctor_Drew]

In other words, is there a way to answer this without the T,x,y diagram?

[@letzfetz1052]

Why is the bubble curve in a pxy Diagramm a line and in Txy Diagramm a curve???

[@marcoszukunft7104]

Hi, Great videos! Let me see if i get it: It does not matter the global composition of the system, the same is gonna happen if we slightly increase the temperature in the region of two phases, both xMeOH and yMeOH will decrease. Now, the composition of the vapor and liquid phases depends of the global composition of the system. If we have a a global composition of zMeOH = 0.2, the vapor phase is going to be more rich in EtOH despite the MeOH is the more volatile component. Am I right?

[@LearnChemE]

Yes, you are correct.

[@mariojuarez2951]

Is the liquid line always on the bottom?

[@LearnChemE]

Great question. It depends on the plot. In this case the liquid line is at the bottom because the temperature is higher at the top (higher temperature is going to move a substance from liquid to vapor). In the case of a pressure diagram, the liquid line would be the top line if the pressure on the y-axis increased from the x-axis to the top of the plot (higher pressure would move a substance from vapor to liquid). Just pay attention to the type of plot and you'll be able to decide which side the liquid is on.

[@sherlockcooper4408]

LearnChemE Thanks a lot!!!

[@charlieni5457]

Hi, thanks for the video. I have a question. Can you explain what's the mole fraction 0.5 means for the first point you chose? Is it the mole fraction of MeOH in liquid phase? I understand when the temperature changes, the mole fraction of each phase will change also, so i'm wondering once the temperature is changed, if we want to know the mole fraction of each phase, should we just use tie line? As you mentioned, when you raised the temperature slightly from the initial point to second point, I can tell the temperature changed from y-axis, but isn't that the mole fraction still staying at 0.5? And how does this describe that when temperature changes, mole fraction changes as well? Thank you very much.

[@LearnChemE]

The mole fraction of 0.5 in the first point is the mole fraction of MeOH in the overall mixture. As the mixture is heated through vapor-liquid equilibrium, the mole fractions of the components are different in the different phases, and yes, you can use the tie lines to find the mole fractions in each phase. The overall mole fraction of MeOH remains at 0.5 and that is clear once the temperature is high enough that everything has vaporized.

[@charlieni5457]

LearnChemE Please correct me if I'm wrong about these following statements: so the first thing we need to know is the mole fraction of MeOH in the overall mixture, and if we want to know the mole fraction of MeOH in each phases, we need to use tie line. And one more question is why the description of x-axis is XMeOH (I assume this is for the mole fraction of MeOH in liquid phase) and YMeOH (the mole fraction of MeOH in vapor phase)? but you also mentioned that the 0.5 is for overall mixture(including liquid and vapor phases), what do we define the meaning of the number? or it just depends on the cases. And thank you for replying me, I appreciate it.

[@LearnChemE]

Charlie Ni Gibbs Phase Rule is used here. We have 2 species (MeOH and EtOH). Thus we have degrees of freedom equal to 4 - # phases present. If we have one phase (all liquid), we need to state 3 other intensive variables to declare the system and determine any other intensive variables. So in this case, pressure, temperature and mole fraction of MeOH would declare our system. If we increase temperature such that two phases form, then we need two intensive variables (pressure and temperature). Composition in both phases is determined by these intensive variables. The tie line (or equilibrium line) can be used to read the composition of both phases.

[@LearnChemE]

Charlie Ni The x-axis is the composition of the more volatile component and is used to read the overall composition of MeOH in the system, as well as the mole fractions in both liquid and vapor phase depending on the line used (dew point line or bubble point line). If no lines are used, its the overall mixture composition.

[@1cabreal1]

Does that mean that some of the methanol in the vapour phase has condensed with the increase of temperature? And thats da reason why the mole fraction of methanol in the vapour phase decreases????? :)

[@LearnChemE]

Thanks for your question. This is an important distinction. No, the methanol is not condensing. In fact, both the ethanol and methanol are evaporating. At the bubble point line, the vapor is mostly methanol, so its mole fraction is very high in the vapor phase. As more methanol and ethanol are evaporated, the increasing quantity of ethanol in the vapor causes the mole fraction of methanol to be lower. However, neither species is condensing as a result of this process.

[@kidding686]

@@LearnChemE If I have a $100 and you have zero and then someone comes along and gives both of us $100. The total percentage of money I have decreases from a 100% to 66%. Although the money in my possession has increased

[@a-levelking8610]

@@kidding686 thats a really good analogy, i think i understand this a little better

[@kidding686]

@@a-levelking8610 Thanks. Glad it helped

[@SandeepGupta-hz3ru]

What is the effect of pressure on txy diagram?

[@LearnChemE]

In a Txy diagram, the pressure is constant. If the pressure changed, that Txy diagram would no longer be valid.

[@tareq155]

this doesnt make sense , where is the methanol going , if it is decreasing in the liquid phase it is increasing in the vapor phase, or else it would be disappearing , please help me figure this out

[@LearnChemE]

We are looking strictly at mole fractions, not mole amounts or mass amounts. When VLE begins, the vapor phase will be enriched in the more volatile component (MeOH). As temperature is increased (or pressure is decreased) both components are transferring from the liquid to vapor phase. Now the mole fraction of MeOH is decreasing in the liquid phase (because it is more volatile) but since water is also transferring, the mole fraction of MeOH in the vapor phase must also be decreasing.

[@tareq155]

thanks so much , i know i got it

[@Doctor_Drew]

Also I don't recall what would be in between the two lines, as in if above blue is vapor and below green is liquid, what is above green but below blue?

[@LearnChemE]

A mixture of liquid and vapor in equilibrium

[@pleumchei.9420]

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