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Dew Pressure


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#1 kezia

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Posted 12 June 2009 - 11:02 AM

I've to find the dew pressure of a ideal mixture of n-butane and iso-butane and the composition of it.
I've got a temperature interval and nothing more.

I tried using an interaction way but I didn't found a solution....
I've got the following equations..

P y1=P* x1
and considering y1+y2=1...

#2 MrShorty

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Posted 12 June 2009 - 12:48 PM

I hope I don't give too much away with this, but here's the basic sequence I would use for this problem:

1) Calculate the partial pressures of the individual components using Raoult's law.
2) Use Dalton's law to calculate the total pressure and the vapor composition of each component.

From there, it's a matter of applying this information to your understanding of what a "dew point" is to obtain your final result(s).

I hope that helps. If you have specific questions about this, post them and we'll try to help you understand.

#3 Zauberberg

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Posted 12 June 2009 - 03:09 PM


Your approach is correct. Dew point - whether observed from a temperature or pressure standpoint - is the fluid state when vapor is in equilibrium with the first liquid droplet formed after applying the pressure on the system. Therefore, as you wrote, Xi*Pi' = Yi*P, and the sum of Xi should be equal to 1. By iterating the system pressure you will reach the solution when both conditions are satisfied.


#4 kezia

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Posted 13 June 2009 - 02:49 AM

first at all, thanks for the anserws...but I'm trying to solve my problem with these equation...

1) by Raoult's law : P=P*1 X1+P*1 X2 where P*i is the partial pressure of the component i

2) Tv1 x1=P*1 and Tv2 x2=P*2

where Tvi is the tension vapour, I calculated it using Datas by Perry, the Chemical handbook

3)P*1=y1 P P*2= y1 P
4) y1+y2=1
5) x1+x2=1
and to risolve the sistem i'm using Excel Solver

I think I found a possible solution...but the for each temperature I found that x1=1 and y1=1 .. is it possible?

#5 MrShorty

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Posted 15 June 2009 - 09:37 AM

1st, let's make sure we know the equations we are using. some of this could be miscommunication.

QUOTE
1) by Raoult's law : P=P*1 X1+P*1 X2 where P*i is the partial pressure of the component i
is not Raoult's law, or you are misusing the term "partial pressure"

QUOTE
2) Tv1 x1=P*1 and Tv2 x2=P*2 where Tvi is the tension vapour
is Raoult's law. If equation 1 is intended to be a different expression of Raoult's law, then it can be derived from equation 2.

QUOTE
3)P*1=y1 P P*2= y1 P
Dalton's law

QUOTE
4) y1+y2=1
5) x1+x2=1
material balance statements for a binary system.

x1=y1=1 is a possible solution, as the pure component point is on the dew point curve. It is kind of a trivial solution, and not a very interesting solution, so I doubt that is the answer we are looking for, but it is a possible solution.

Gibb's phase rule (df=2+NP-NC where df= degrees of freedom, NP=number of phases, NC=number of components) for a binary system tells us that we have 2 degrees of freedom for a binary VLE problem: which means we have to specify two variables to solve the problems. From your last statement, it appears you are fixing T and P, and trying to solve for x and y.

At this point, I'm not sure what kind of algorithm you are using to solve the system of equations. Excel Solver should be a good tool to solve this, but a lot will depend on how well you set up the spreadsheet. At this point, I'm not sure how best to help you further. If you will describe in more detail how you are trying to solve the equations, I can maybe point out other problems.

#6 kezia

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Posted 24 June 2009 - 12:21 PM

first at all thanks...

I used Excel Solver.
my aim cell is y1+y2=1
and my bonds are the others equation
and I change y1-y2-x1-x2 partial pressures and dew pressure.
and I don't know how to find the other solution....

and i've to modify equations considering non-ideal mix...
how can I do it??

I've to compare my results to the ones i find modeling the complete system using an appropriate equation of state.

#7 MrShorty

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Posted 25 June 2009 - 09:31 AM

I'm still not entirely sure how you have your spreadsheet set up.

Again, I hope I'm not giving away too much so that I'm interfering with your learning process. As noted above, here's how I'd solve this problem:

select T and x

QUOTE
1) Calculate the partial pressures of the individual components using Raoult's law.
(P*i)=Tvi*xi where the quantity P*i is the partial pressure of component i, Tvi is the vapor pressure of pure i at the selected temperature, and xi is the selected liquid composition. Then:

QUOTE
2) Use Dalton's law to calculate the total pressure and the vapor composition of each component.
Ptot=sum(P*i) and then yi=(P*i)/Ptot.

At this point we have calculated total pressure and vapor composition at a given temperature and liquid composition. You simply need to apply this information to the specific question at hand (dew point).

Does that help?

#8 kezia

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Posted 26 June 2009 - 08:02 AM

Ok..but my problem is...
I know only the temperature of my system.
And I used Perry's formula to calculate pressure vapour at that temperature..
considering ideal mix in the first case, and non ideal mix in the second case

#9 Zauberberg

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Posted 26 June 2009 - 11:32 AM


Kezia, you just have to read my post again, and a little bit more carefully. All the answers are there. No reason to re-invent the hot water.

Good luck cool.gif

#10 kezia

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Posted 28 June 2009 - 07:36 AM

ok thanks so much, I'll try to interact another time and I hope it works... biggrin.gif


#11 MrShorty

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Posted 30 June 2009 - 01:24 PM

QUOTE (kezia @ Jun 26 2009, 07:02 AM) <{POST_SNAPBACK}>
Ok..but my problem is...
I know only the temperature of my system.
good. What else do you know? I'm guessing you also know the vapor/gas composition?
QUOTE
And I used Perry's formula to calculate pressure vapour at that temperature..
good
QUOTE
considering ideal mix in the first case, and non ideal mix in the second case

So, you have T and, I assume, y, and need to calculate P and x, is that right? I think at this point, in order to be most helpful, we need to know exactly what you are stuck on. What can you calculate, what can't you calculate?

In many ways, the only difference between the ideal and non-ideal cases is the introduction of a "correction factor." Once you figure out how to do the ideal case, the non-ideal case will use the same basic procedure, you will just add in the "correction factor" (activity coefficient, for example, if you are going to use an activity coefficient equation to calculate how it deviates from ideal).





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