5 replies to this topic

[Mahesh@A&M]

Can someone please guide me on partial pressure calculation for this applicaion.

I have a tank filled with acetone-polymer mixture(for simplicity I assume 100% acetone) and pressurized to 500 psig with hydrogen. What would be the partial pressure of hydrogen in the tank when the temperature is raised to 150 deg F. Pressure is maintained at 500 psig with a control valve.

Appreciate your help.

[breizh]

Hi ,
H2 ?
Pt = total pressure = Pv acetone +P H2 = (500 +14,7) PSI = 3548 Kpa
Vp acetone at 150F ( 65.55 C) = 136 Kpa

Hope it helps
Breizh

[siretb]

At 150°F the vapor pressure of acetone is about 20 psi

The total pressure is 515 psi(a) to H2 partial pressure will be 495 psi

[mbeychok]

I believe that you might find it useful to read this article in the online encyclopedia Citizendium:

Partial pressure

[aluma]

Can someone please guide me on partial pressure calculation for this applicaion.

I have a tank filled with acetone-polymer mixture(for simplicity I assume 100% acetone) and pressurized to 500 psig with hydrogen. What would be the partial pressure of hydrogen in the tank when the temperature is raised to 150 deg F. Pressure is maintained at 500 psig with a control valve.

Appreciate your help.

Hi Chemroopa,

You would first of all have to compare the vapour pressure of acetone at 150 deg F to the Total pressure of the system. At 150 deg F (338.706 K), the Pv of acetone is 19.96 psia or 5.26 psig. Since this vapour pressure is << than the system pressure, it is safe to assume that acetone is hardly in the vapour phase.

According to Dalton's law of partial pressure, hydrogen partial pressure would thus equal the system total pressure. Since pressure is constant, hydrogen partial pressure = 500 psig.

[MrShorty]

I don't know if you are still interested in this problem. I'll throw my thoughts in here. But first, I would say that it depends on how rigorously/exactly you need to know the partial pressure of acetone.

In many cases, I would probably use siretb's approach: essentially assume an ideal mixture and that the H2 is insoluble so that the partial pressure of acetone is simply the vapor pressure of pure acetone. This is a computationally easy approach that sometimes yields adequate results. Where you are already making other simplifying assumptions (such as polymer concentration is 0 and H2 solubility is 0, so that you can assume the liquid is pure acetone), you probably don't feel you need a rigorous solution.

Aluma did essentially the same thing, but after seeing that the vapor pressure of acetone was less than some threshold (in this case, vapor pressure of acetone is about 4% of the total pressure), he decided to simply ignore the partial pressure of acetone (in essence, Pacetone=0).

To enhance the discussion (or maybe to unnecessarily complicate the discussion, depending on your point of view), I'll point out that, even at 500 psia, the phenomenon sometimes called "enhancement" may be important. To illustrate "enhancement" I'll refer to some data for hydrogen and n-hexane reported by Nichols et al (AICHE J volume 3 p 262). They report VLE data for H2 and hexane at about 378 K. They report a vapor pressure for pure hexane as about 2.7 bar. With H2 added so the total pressure is about 35 bar (or about 500 psia), they report a partial pressure for hexane of about 3.3 bar -- roughly 20% higher than the simple calculation proposed by siretb.

A rigorous calculation for this problem would involve using an equation of state. Clearly this will be much more complex than the simple approaches already proposed.

As noted earlier, even large errors in the calculated partial pressure of acetone yield small errors in the partial pressure of H2. If P(H2) is the only important quantity, then it still probably doesn't matter if we go through all the effort to calculate any enhancement for the acetone. However, if at any time you feel that the partial pressure of acetone becomes important, you may want to consider a more rigorous calculation.

Edited by MrShorty, 01 October 2009 - 10:55 AM.