# Evaporation of water content from a solid material by applying low pressure

I have a raw material which melts at $96\ ^\circ C$. My aim is to make water content evaporate at temperature below this temperature. I can apply vaccume oven for this. I want to know at what pressure the water evaporates by keeping low temperature so that the material doesn't melt.

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Related: physics.stackexchange.com/a/25402/520. But if there is a mixture then the phases diagram there is not exactly right; see Pete's comment on my answer there. – dmckee Jan 7 '13 at 14:02
I think water evaporates even at room temperature but slowly. The boiling point of water can go below $96\ ^\circ C$ if its vapor pressure drops below 0.86 atm. – Tariq Jan 7 '13 at 17:01

## 1 Answer

Water will evaporate as long as the relative humidity of the surrounding air is below 100%, although the process may be (very) slow. You seem to be confusing evaporation with boiling, but you don't need to boil the water in the material to dry it.

It depends a lot on your configuration, but you are probably better off blowing hot dry air over your material than creating a vacuum, because then water vapor molecules will be moving away from the surface of your material by diffusion, which is normally a much slower process than convection.

Still, if you want to boil the water content of your material, your guide should be the vapor pressure of water at your temperature of choice, which you can check here. As an example, the vapor pressure at $90\ ^\circ\mathrm{C}$ is $70\ \mathrm{kPa}$, so if you heat your sample to that temperature and have a vacuum of less than $0.7\ \mathrm{atm}$, there will be boiling.

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