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Potable Water - The Third Way.
Mark Borgerson wrote:
In article , says... SNIP You need to get back to the gas law to see where this error lies. You have to *create* the vacuum. That requires a HUGE increase in volume for whatever the initial headspace is. For this to happen you need a much longer tube to start with. You seem to have missed the fact that I proposed filling the tubes completely with water so that the initial head space would be zero. No, it won't be zero. It can't be. If it is, then you have a solid liquid stream, and it's just a siphon. You have to have headspace. And it has to be sufficient to maintain separation of the seawater and freshwater to prevent contamination when filling the tubes. And it has to be large enough to prevent percolation carryover when boiling is initiated. At that point you release the pressure on the water and it falls to the point where water weight plus vapor pressure equals 1ATm. A solid liquid loop will not separate into two separate columns. They have to be separated by a headspace. You can heat the seawater side and create a headspace by liberating dissolved gases, then let the columns drop to create vacuum, but you will have contaminated the freshwater side. At that point, you essentially have two water barometers, interconnected at the top. One is salty and warm, and one is fresh and cold. Neither need be too much longer than 33 feet. The actual height of the water will be less than 32 feet by a factor dependent on the temperature of the water in the warm side. The real practical problem lies in the addition of the dissolved gases in the seawater to the water vapor in the headspace. What we have here is a rather inefficient degassing column. I spent a lot of time degassing seawater while working on my MS in chemical oceanography. I was trying to measure the dissolved hydrogen in seawater, and the oxygen, nitrogen, methane, and other gases kept getting in the way! Getting rid of the disssolved gases in the headspace and as bubbles forming on the sides of the tube is going to be a major headache. Not a headache, an impossibility (they're not really dissolved at that point though) :-) That, and the increase in pressure due to water vapor will make this an oscillating, self-quenching system. It'll require more and more heat as the partial pressures of the non-condensables increases, and the column heights will drop as the pressure goes up, with the diffusion path increasing the whole time. Keith Hughes |
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