Water distillation under vacuum

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Artybee
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Water distillation under vacuum

Post by Artybee » Wed Aug 16, 2017 7:49 pm

Thoughts on this idea?
One method of making water safe to drink is by distillation. This both used heat which kills biological contaminants, and evaporation/condensation which separates the water from non-biological contaminants.

I have an idea to do this under a lower pressure system, so that the water evaporates at a lower temperature, this using less fuel.

I alm considering two glass demijohn (carbuoys) connected by tubing, with the means to reduce the pressure inside by means of a hand pump.

I have a few questions on this set-up:
0. Is this workable, or is there some fundamental flaw that I haven't realised?
1. Will reduced pressure affect the condensation point, thereby making this a pointless exercise?
2. Can sufficient vacuum be obtained by means of a hand pump to make this effective?
3. Will the distilled product still be safe with lower heat input?
4. Could it work on a solar basis with the primary container being in the sun and the receiver being in the shade?

Thanks for any input on this.

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Jamesey1981
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Re: Water distillation under vacuum

Post by Jamesey1981 » Wed Aug 16, 2017 10:08 pm

The problem you will have is with maintaining the vacuum, it may also affect the condensation point but I'm not sure of that, (edit: thinking about it, it probably won't condense at all if you manage to get the pressure below the vapour pressure of water at the temperature you're trying to condense it) but as the water evaporates and turns into water vapour it will expand, thus increasing the pressure in your container.
You could keep pumping and maybe you could keep it under vacuum, but even if you manage it, you're going to be removing gas from your system, which will include the water vapour that you're trying to condense.
So in summary, if you have a vacuum you won't have any vapour to condense, and if you don't have a vacuum you're wasting your effort pumping.

Artybee
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Re: Water distillation under vacuum

Post by Artybee » Thu Aug 17, 2017 7:32 am

Jamesey1981 wrote:but as the water evaporates and turns into water vapour it will expand, thus increasing the pressure in your container.
You are right; I had completely forgotten about expansion. I guess that rules out using a sealed system.

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sniper 55
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Re: Water distillation under vacuum

Post by sniper 55 » Thu Aug 17, 2017 7:57 am

I'm no expert but wont demijohns collapse under the vacum? I've seen metal 55 gallon oil drums crushed by just lowering the pressure inside, and that wasn't even close to a vacum.

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oldman
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Re: Water distillation under vacuum

Post by oldman » Thu Aug 17, 2017 12:45 pm

Hi,

Using a vacuum will not work as the water vapour will fill the vacuum.
sniper 55 wrote:I'm no expert but wont demijohns collapse under the vacum? I've seen metal 55 gallon oil drums crushed by just lowering the pressure inside, and that wasn't even close to a vacum.
I think the danger here is that the steam pressure will burst the demijohn as it is a sealed system, think about the power of a steam engine.

If you can create a vacuum then a better use would be to draw the water through a filter system, in the same way you suck water through one of those water purifying straws.
"Just when one least expects it, the unexpected always happens" - Dr. Rance

Artybee
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Re: Water distillation under vacuum

Post by Artybee » Thu Aug 17, 2017 1:01 pm

All valuable points.

I was working on the assumption that a partial vacuum would make it easier for the water to vapourise. But I think a side effect is that it would be less likely to condense.

The system I was thinking of would use rubber bungs which would easily blow off if the steam pressure was too high.

Having looked in to this a bit more, it seems that a hand pump can achieve at the most 0.8 atm which leads to a boiling point reduction to about 94°C ... not really worth pursuing.

preparedsurrey
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Re: Water distillation under vacuum

Post by preparedsurrey » Thu Aug 17, 2017 4:56 pm

If you lower the boiling point significantly then you will not kill the bugs and nasties. If I recall correctly water needs to be held at a rolling boil (under normal pressure) for a length of time to ensure bugs are killed.
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ForgeCorvus
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Re: Water distillation under vacuum

Post by ForgeCorvus » Sat Aug 19, 2017 4:28 pm

Surrey: Not exactly, but near enough
The minimum temperature recommended to kill nasties is 70 degrees C, however this needs to be maintained for a while to be affective and is difficult to guess in the field.
Everyone can tell when water is at a Rolling boil and holding at that point for a minute or two ensures that the water has been over the magic 70C for long enough to kill anything thats likely to affect human health.

Even on the top of Everest water boils at over 70C...... Not by much though :ugeek:
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