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Author Topic: Can water be distilled with a vacuum pump?  (Read 2953 times)

Offline McKay

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Can water be distilled with a vacuum pump?
« on: 01/06/2014 01:59:43 »
Greetings.
A thought experiment of mine is keeping me awake (and I am bad at actually creating something and testing in a garage or something):
We know that decreasing pressure for water makes its boiling point lower/ makes it evaporate more. Using a handheld vacuum pump (perhaps a modified cylinder bicycle pump, reversing the valves) with a bit of water and nothing else in the main cylinder, bottom (intake for the modified bicycle pump, output for the original) valve closed, one pulls up on the handle, decreasing the water pressure by making the volume expand - water vapor is created in the now expanded volume of the cylinder. Now, if the handle is allowed to fall back again, with the one way valve allowing that newly created water vapor to escape - isnt that distilled water coming out of the pumps what-used-to-be-air-intake?

How efficient could such a simple pump for distillation/ desaltification be?
Hope you understand what I mean.
« Last Edit: 05/06/2014 11:12:25 by chris »


 

Offline CliffordK

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Re: Water distillation with a vacuum pump (?)
« Reply #1 on: 01/06/2014 20:03:55 »
I've wondered about this a bit myself. 

A vacuum pump is apparently a useful aid in distillation of a minor constituent out of a tank with a second compound with a significantly different boiling point such as water out of biodiesel.

As far as bulk distillation, it may be efficient, if one could use all the resources available. 

Two designs would be possible. 
  • Equal pressure between evaporator, and condenser, but temperature differential.  The temperature differential must be maintained somehow.  Solar heating?
  • Equal temperature between evaporator and condenser, but actively pump between the two.
Minimizing pressure/temperature/energy differentials would be vital for efficiency.  I'm not sure about the speed of low pressure distillation.  Would it be similar to having a pot on the stove at 100C, 1 ATM, adding some heat?

For example, design your system with the discharge tank being 34 feet deep.  There is a risk of evaporative cooling, and heating at the condenser, so perhaps the two tanks should either be joined, or otherwise make a heat exchange.



You would need to deal with purging the brine and evacuating the air.  But, the system should be fairly efficient.  I don't know if it could be easily expanded to the capacity to serve a city the size of Dubai, or a large plantation, but perhaps one could easily distill enough water for a small village, or small personal farm.

One could probably design a tidal pump to resupply your still. 
 

Offline CliffordK

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Re: Water distillation with a vacuum pump (?)
« Reply #2 on: 05/06/2014 07:15:52 »
I've thought about this a little more.

If you had a semi-permeable membrane, there would be osmotic pressure to force the pure water to mix with the salt water. 

Likewise, I think if you had two chambers connected that were sealed either under 1ATM, or a vacuum, there simple evaporation/condensation would favor movement from the pure water chamber to the salt chamber.  Thus, you need to force a gradient in the desired direction, either with an active pump, or using a temperature gradient.
 

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Re: Water distillation with a vacuum pump (?)
« Reply #2 on: 05/06/2014 07:15:52 »

 

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