Supercritical Drying with Water

[NateTG]

Do people do supercritical drying with water?

The critical temperature and pressure of water - about 375 C and 3500 PSI - don't really seem that high. Some silica aerogels apparently handle temperatures that are much higher than that, and typical oxygen bottles are at 2500 PSI so it seems like it should be possible to make aerogel with water as the drying gel solvent.

 

[Fearless]

Hmm, I think it's really a matter of the heat capacity. The Cx (x = P or V) is really very much higher in water than in many industrial chemicals. Besides, to warm a liquid to about 650K is _very_ expensive. I wouldn't do it if the industry was about earning top dollar.

But my main concern is the pressure. Man, that is some really expensive thing you are about to build if your P = 250 bar. It will cost you dearly in manpower and time.

Sure, you will probably not do it at 250 bar, but even at 180 bar it will cost you a nickel and a dime.

It may be doable, but probably not economical.

 

[Astronuc]

Do people do supercritical drying with water?

The critical temperature and pressure of water - about 375 C and 3500 PSI - don't really seem that high. Some silica aerogels apparently handle temperatures that are much higher than that, and typical oxygen bottles are at 2500 PSI so it seems like it should be possible to make aerogel with water as the drying gel solvent.

I'm not sure what the question is asking. Drying (e.g. of air) is usually done by condensing water.

If one is referring to dry steam, then yes there are some superheated systems, and there is one advanced nuclear reactor design based on supercritical conditions. It is however technically challenging!

With respect to oxygen (or most gases) bottles at 2500 psi, those are usually at room or low temperature.

The challenge is to find a material that has high strength for high pressure (3500 psi) at that temperature (375 C). The pressure vessel must be designed and manufacture to ASME B&PV (or equivalent) specifications. The structure must be free of flaws above a certain critical dimension, and the material must have low corrosion rates over some reasonable lifetime, e.g. 15-30 years.

 

[NateTG]

I'm not sure what the question is asking. Drying (e.g. of air) is usually done by condensing water.

I'm interested in aerogel formation. Currently the processes seem to involve replacing the gel solvent with liquid carbon dioxide, and I was wondering if anyone had made a silica water gel, and supercritically dried it without going through the diffusion replacement process.

 

[neutralino137]

According to the ever-reputable Wikipedia, water becomes a potent oxidizer when it goes supercritical. This likely damages the material you're trying to dry more than simply drying by evaporation (which can cause damage because of surface tension) or freeze-drying (which leaves nice pores).