5 replies to this topic

[kezia]

There are n-butane and methylpropane in equilibrium.
I've find the equilibrium costant in a temperature interval and, then, I calculated y1 and y2 so I found even the dew point event the liquid composition.

Now I've to calculate the Bubble Point and I know the temperature, tension vapour.
I need to know X1 and X2, can i use the same I found before???

[rgmmelo]

Correct me if I'm wrong; I didn't understand very well , but you assumed a temperature and pressure and then calculate a vapour composition? Now, knowing the composition, you assumed another T or P to calculate dew T or dew P? if was that, you can use liquid composition to calculate bubble point. But you should use a ifferent T or P from that used in the VLE. Otherwise, you going to do the VLE calculation again.

Apologies! My English isn't so good yet.

[kezia]

never mind, my english isn't very good too!

well...
in the first point of my exercise I had to find dew pressure.
So I found:
1) equilibrium costant
2) vapour composition (K=y1/y2) stechiometrical coefficients are one
3) using Tension Vapour, i found Dew Pressure and xi (<-- liquid composition)

Now I've to calculated Bubble pressure.
I use the following equation:

y1*P=x1*Tv1

but i need to know x1 and x2 but if i can use the same i found before...

[rgmmelo]

never mind, my english isn't very good too!

well...
in the first point of my exercise I had to find dew pressure.
So I found:
1) equilibrium costant
2) vapour composition (K=y1/y2) stechiometrical coefficients are one
3) using Tension Vapour, i found Dew Pressure and xi (<-- liquid composition)

Now I've to calculated Bubble pressure.
I use the following equation:

y1*P=x1*Tv1

but i need to know x1 and x2 but if i can use the same i found before...

you can use the x1 and x2, but in the diferents conditions that you used in VLE( a different T). If you want, post the complete question Here.


Ps:I dont know this method that use "Tension Vapour" to calculate anything in VL equilibrium.

[MrShorty]

rgmmelo: Vapour Tension is an older term for vapor pressure.

kezia: Whether you can use the xi's you calculated in the previous problem for the bubble point of the new problem depends greatly on how the problem is worded. I hope it is clear that the pressure and liquid composition you calculated in the previous dew point problem does represent a point on the bubble point curve. Whether or not it represents the bubble point you are being asked about in this part of the problem depends on how the problem is worded. Intuitively, I doubt it would be, but I can't be certain.

I don't know if you've tried this, but I often find these kinds of problems easier to understand if I sketch out the phase diagram. This way I can visualize the point that I'm being asked to calculate in relation to the rest of the phase diagram.

[kezia]

Sorry, I didn't explain very well so..
I've to solve this exercise (n-butane <-> iso-butane in equilibrium), it developes in this way:
Consider a range of temperature between 25°C and 500°C ( 298,15K and 773,15 k) calculated:

1-how change equilibrium costant due to temperature; (i used van't hoff)
1.1- calculated liquid satured composition and dew pressure;
1.2- calculated bubble pressure.

then there are other points but i solved it without problems using fugacity coefficient.

i hope that now it's more understandable