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Henry's Law question

Henry's Law question

Henry's Law question

Be gentle, I am not a chemical engineer.

A cylindrical vial with a known, internal volume is filled with an unknown (propriatary) fluid. There is a small air pocket left in the vial before it is sealed off. This vial is then loaded, in compression, with a large enough force to cause deformation of ~.005" in the length of the cylindrical vial. This force is not large enough to cause yielding of the vial, when the load is removed the vial returns to its original shape.

Question 1)
According to Henry's law some of the gas (air) in the vial will be dissolved into the fluid when the vial is compressed, correct?

Question 2)
When the compressive load is removed will the gas that dissolved into the liquid remain dissolved in the liquid?

Question 3)
Does the answer to question 2 depend upon the fluid?

That is really all of the info. I can provide. I appreciate all responses.

Thank you!

RE: Henry's Law question

The equilibrium states before and after compression are the same, but time is needed to reach equilibrium. Air contains oxygen, which is a very reactive gas. Most vials are therefore purged with nitrogen before sealing.

RE: Henry's Law question

PA = xAHA(T) ---> xA = PA/HA(T)

1) Yes. Some will be dissolved before the vial is compressed, just from whatever pressure is there before compressing. Compressing will increase the amount dissolved.

2) No. Only what was dissolved in the fluid before the compressing.

3) Yes, if the air (oxygen, nitrogen, carbon dioxide, etc.) reacts or bonds with the fluid it may not be reversible.

Good luck,

To a ChE, the glass is always full - 1/2 air and 1/2 water.

RE: Henry's Law question

Over geological time Compositepro is exactly correct. Probably over a few days he is still correct. I've found super-saturated (after a pressure change) conditions can last for several hours after the pressure is reduced. The answer to Question 2 is maybe, maybe not for long, maybe not. I've done a lot of research (both literature searches and some experimentation) on this for a lawsuit where I was engaged as an expert witness, and the timing for equilibrium seems to be a complete crap shoot. I could not find any consensus on the mechanism of disengagement let alone the timing.

The funniest thing I found in reading upwards of 50 Masters and Doctoral thesis is that they didn't agree on the DIRECTION of the change in the Henry's Law constant with changes in temperature (they all agreed that the constant increased with increasing pressure, but they disagreed on the rate of change, for temperature they couldn't even agree on the direction). Probably 1/3 said the constant followed temperature and 1/4 said it went in the opposite direction and nearly half skirted that issue altogether. Dissolving gases in liquids is anything but a settled science.

Finding reliable Henry's Law constants for solvents other than water is a pretty major undertaking. You can find limited data on some common solvents, but the results are very inconsistent from one researcher to the next. The magnitude changes quite dramatically from solvent to solvent, so I think your only chance with a proprietary solvent is to see if the manufacturer has done the experiments.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

RE: Henry's Law question

For a particular gas dissolved in a particular liquid, there is a fixed solubility at any given temperature and pressure. The solubility of gasses in liquids increases with increasing pressure and decreases with increasing temperature. That is indeed a settled science. The specific solubility for any given gas in a given liquid at a given T & P is fixed by the laws of nature. The hard part is for us human apes to figure out what that value is. Thus the wild variability that David accurately describes. Although the solubility of a gas in a liquid at a particular T & P is fixed, you'll find wildly varying solubility values in technical literature. That doesn't mean that that the value is indeterminate, it just means that we have a hard time with experimentally determining the true value.

So, the answers by Latexman are correct. The solubility of a given gas in a given liquid, at a given T & P is fixed. When you remove the pressure, the system will revert to it's equilibrium state. The time it take to do so is dependent on the diffusion coefficient, but the equilibrium concentration of the gas in the liquid is a fixed value. OK, but what is this fixed concentration value? As David points out, that is hard to know, because it's difficult for us humans to get an accurate measurement of that concentration value. Thus the wide variation in Henry constants found in technical literature.

RE: Henry's Law question

Henry's constants for sparingly soluble gases in liquids are accurate enough, but Henry's constants for sparingly soluble organics in water are TERRIBLE. By definition, the constant is simply the vapour pressure divided by the solubility, but you'll find Henry's constants which vary by orders of magnitude in the literature.

As to the answer to the original question, it all depends on whether or not the liquid in question was already saturated at ambient conditions prior to being put in the vial. That should not be assumed unless it has been stored for a very long time in contact with air or it has been aggressively contacted with air in the recent past. If the initial liquid is subsaturated, it may end up being merely saturated or even subsaturated after the vial is opened again.

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