5 replies to this topic

[irongut]

Here's the problem:

A stream containing equimolar quantities of benzene, toluene, and ortho-xylene is fed into a flash separator.

a. If the flash separator operates at 50 ˚C, what are the minimum and maximum pressures (in kPa) where there is a two-phase region (liquid and vapor)?

(A: Max P =20 kPa, Min = 7 kPa)

So here's the approach I took:

If our species are equimolar in the feed, then assuming we have say, a 100 mol/s feed, we'd have mole fractions of each species =0.333

Now, I found the partial pressures of each species using the Antoine equation and the given A,B, and C values from my textbook:

Benzene: A=6.89272 B=1203.531 C=219.888 ==> p*=271.328 mmHg

Toluene: A=6.95805 B=1346.773 C=219.693 ==>p*=92.1128 mmHg

o-Xylene: A=7.00154 B=1476.393 C=213.872 ==>p*=25.4935 mmHg

Now with my understanding of the question, we're supposed to find the range of pressure values for which the entire system will be in the two phase region as opposed to just when the species themselves will enter the two phase region.

So with that understanding, I went to find the bubble pressure of the system:

P(bubble)=0.333271.238+0.33392.1128+0.333*25.4935 ==>P(bubble)=129.602 mmHg

And since my professor asked for it in kPa:

129.602(101.3kPa/760mmHg)=17.27kPa, which is substantially lower than the 20kPa in the given answer above.

Any ideas what I'm doing wrong here?

[breizh]

Hi ,

 

The region you should cover is between dew point and bubble point ( 2 phases) .

 

Hope this helps

 

Breizh

Edited by breizh, 26 February 2016 - 03:44 AM.

[MrShorty]

If it helps, I get the same answer for the bubble point as you get. I note that you are assuming Raoult's law for this calculation. Is it possible that your professor expects you to incorporate some measure of non-ideality in your calculations (an activity coefficient, for example)? Is it possible that the textbook's answer is rounded to one significant digit (17 would round up to 20)?

[breizh]

Hi irongut,

 

http://www.questcons...dew_points.html

 

You may find useful the link above to validate your calculation. Result is similar to yours using the link (17.7 kpa for bubble point) .

 

Breizh

[OBASSI]

Hello Everyone. I am trying to size a separatorwith 2 identical separation trains. The fluid from the manifold enters each of the two HP separators and is stabilised by flashing it through intermediate and low pressure separators (3 stage flashing) for each train. the HP separator has 50% liquid fill and 10% of the liquid is entrained in the gas stream during flashing. With an oil feed rate of 3650 and vapour feed rate per train, liquid droplet size of 150, what type of separator would give the best phase separation?

[shantanuk100]

Hello Irongut,

 

There's one other possibility.

The antoine equation is valid, but not exact over the entire pure component range.

It varies considerably for components when we cross over near the triple points, critical points etc, so you cannot use the same A,B,C values over the entire range of the component, but have to change it when you cross over.

 

Instead, Try using the Lee Kesler method to estimate the component vapour pressures and then see if the total pressure varies.

It is much more accurate over the entire range and I think it should make a difference considerably. You probably would get 20 kpa with that.

 

Regards,

Shantanu