6 replies to this topic

[birdycofir]

Hi, I'm a 2^nd year Chemical Engineering student and I was stuck on this question.  The question is on the left and the solution is on the right.  Basically, I just don't know how to start or how to use Excel to solve this.

1. What is the difference between Pj and Pc,j?  What is the relevance of the j?

2. What is χj?  I'm guessing it's mole fraction.

3. In the solution, why is temperature assumed to be 320 K?

[Jamal Khan]

Hi birdycofir

I will be a little honest here. The manner in which you have posted the questions clearly suggest that you haven't listened to your professor clearly during the lectures and maybe you haven't referred any book before actually solving the questions. I advice you to first understand the Raoult's law, the antoine equation (before understanding wagner's equation) critical Pressure and Temperature and refer a book. I dont think you would learn anything if somebody merely gives you the solution of your problem.I suggest Van ness and smith, its a good book to understand vapor-liquid equilibria. After that If you don't understand something feel free to post about it. Pardon me If I sounded harsh.I am just trying to help you.

Edited by Jamal Khan, 13 April 2016 - 03:25 AM.

[breizh]

birdycofir,

We are not doing homework but we are always trying to help people . Show us your difficulties and we surely will support you.

The query about point 3 is really weird : it's written . The graph is a Function of ( T,P) ,

The only way to understand is to write the equations , the solution depends on T , with y1+y2=1 . 320k is to initiate the calculation , it is an iterative calculation.

 

note :

Jamal kahn is absolutely right  ! 

 

Breizh

Edited by breizh, 13 April 2016 - 04:23 AM.

[birdycofir]

Thanks for your feedback guys.

 

Jamal, I did email my lecturer previously and he told me the same thing about not focusing in class. That is the reason I am here. I am not too keen on getting textbooks since I only have 2 months of second year to go, but perhaps now I may consider it.

 

Breizh, what do you mean by "it's written"? Written where? What graph are you referring to? I only see the graph of y against x in the solution. Do you mean that 320K is just some kind of estimated starting point to solve the problem?

[breizh]

birdycofir ,

 

Yes , T = 320 K is a guess , to start with ........> y1 and y2 are fonction of (T,P) , and you must iterate on T to get y1+y2= 1 !

 

Hope this is clear now

 

Good luck

 

Breizh

Edited by breizh, 13 April 2016 - 05:39 AM.

[MrShorty]

I will agree that you have obviously missed some basic principles in class, but I like how you have identified what principles you have missed and asked those specific questions. That is worth something, IMO. If I may attempt to contribute to answering these questions.

 

1) What is j? j is simply a quick way to designate "compound". In the context of this problem, j refers either to "heptane" or to "octane".

Pj is the vapor pressure of pure octane or heptane, and Pc,j is the critical pressure of octane or heptane.

 

2) Xj? Which xj are you referring to? Your problem defines two Xj's. In the Wagner equation, Xj does not refer to mole fraction, but refers to the quantity 1-T/Tc for the Wagner equation. In part 2, we are indeed using xj to refer to mole fraction. You will need to keep track of which xj you are referring to as you solve the problem.

 

3) As others have noted, T=320 is given as a starting point for solving for T at P=1 atm for each VLE point. You could just as easily have picked 300 or 350 or whatever you felt like. Since I know that, for an ideal solution (assumption made in the problem statement), the boiliing points of the mixtures will be between the boiling points of the pure components, I probably would have picked a starting point closer to the boiling points of octane and heptane, but it makes little difference.

 

Does that help answer your questions?

[birdycofir]

Thank you MrShorty, that was very helpful. It was just those little things I needed clarification on.