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Potable Water - The Third Way.
Mark Borgerson wrote:
In article , says... snip Yes, and this "migration" is simple diffusion. *And* you have (in the example above) 33' of column it has to diffuse through on the seawater side, and however many feet of column on the freshwater side it has to traverse prior to condensation. If both columns (fresh and sea) are referenced to the same height, then the evacuated column height on both sides will be the same, and that diffusion path will be up to 66'. That does not happen quickly. How do you get 33' as 1/2 of the diffusion path. A quick thumbnail guesstimation at where equilibrium would likely be reached. I didn't take the time to calculate the exact heights. I think there will be about 33 feet of water in the column on each side Then I think you would be wrong, unless your columns are significantly longer than that, probably more like 50+ feet. ---to provide the weigth that pulls the pressure down. That would leave only about 7 feet of water vapor path on each side of the column. There is no vacuum to hold the water up - the vacuum is what you are trying to *create*. The water columns will drop until there is an equilibrium point reached between the external atmospheric pressure, the height (weight as you state) of the water column, and the pressure in the headspace (the U-tube). The water columns *must* retreat, or the headspace stays at atmospheric pressure. If the tubes are long enough, and the initial column heights are high enough, then when you reach equilibrium, you'll have close to a vacuum and close to 33' water column heights. And a lot more empty headspace than you started with. Use the ideal gas law: PV=nRT For our evacuation purposes, nRT is a constant (#moles is constant, R doesn't change, and assume constant temperature), so if you start with a volume of 1 liter, and a pressure of 14.7 psia, and you want to reduce that pressure to 1.47psia, then you need a 10-fold volume increase. You want to reduce it to 0.147psia? then you need a 100-fold initial-volume increase. I'm not sure that 'diffusion' is the proper term for the motion of the water vapor. After all, the heat engine is providing water vapor on one side and condensing it on the other---so there is a net mass flow and probably a small pressure differential to move the vapor. Well, diffusion is the primary mechanism. What happens when your 'heat engine' creates water vapor? It doesn't just immediately condense on the other side. It creates pressure on the heating side, which does two things. One, it drives both the water columns *downward*, and it raises the boiling point on the seawater side (it does, however, make condensation on the fresh side more efficient as well). You can't look at this as a static system where the pressure stays the same or the column heights stay the same. It's a dynamic system, and will reach an equilibrium point with the columns much lower than the initial starting point, and the headspace pressure much higher. And don't forget, there will also be significant evaporation (due to low partial pressures) on the freshwater side that will be in equilibrium with (and in opposition to) the condensation process. It's not as simple a system as it seems. That's why this system *will* work, but it must work very slowly. Still (pun intended), you need a lot of heat to provide the energy to evaporate the water or it will soon cool to the point where its vapor pressure is reduced and the process slows drastically. My 'guess' would be that the system would end up operating around 4-5psia when equilibrium is reached, which would require a temp of about 60°C (140°F) to maintain boiling. Here in my neck of the woods, our energy from the sun ranges from about 220-360 BTU/ft^2/Hr measured at normal incidence, depending on the time of year. A couple of decades ago I worked at a solar test lab and we tested all kinds of collectors, including swimming pool collectors which are unglazed (i.e. no cover over them to exclude wind). Bare copper tubes, painted black, with no wind, are about 15% efficient at solar absorption (#'s are from my old memory, so...) when the tubings' longitudinal surface is perpendicular to the incident angle. However, with a 3 mph wind (per ASHRAE 95-1981 which we used for indoor system simulations) that efficiency drops to the low single digits. When you factor in off-angle response (i.e. since the tubes won't be on a tracking mount to keep them 'aimed at the sun") the basic efficiency drops from ~15% to probably ~8%, and with the wind, between -3% to 3%. So, using only the tube as a collector is a real challenge. Probably be better using a flat-plate collector as the primary heater, but that's another major addition to the complexity. Of course, too much heat would kill the system with over pressurization. The fact that the water 'boils' near room temperature does not reduce the amount of heat required to change the water from liquid to vapor. No, in fact the lower pressure raises it a bit. Latent Heat of Vaporization for water is inversely proportional to the pressure, albeit the change is less than 10% IIRC. As has been discussed, the simple idea does not address the problems of salt buildup in the seawater side, or the addition of dissolved gasses to the vacuum part of the loop. Non-condensables are a rate limiter for the process, unless you want to spend more energy for vacuum deaeration. With a large enough (or double) sal****er tube you might get a convection cell going with the cold, saltier water sinking and pulling up warmer seawater to the top. Certainly possible, but not easily doable. You could solve the dissolved gas problem by periodically pumping both tubes up enough to displace the accumulated gases. Well, if you added a convection cell as above (another system that requires time to reach an equilibrium condition to work), then the periodic headspace purging would quench both the distillation and the seawater convection systems. In reality, the purging would be likely be very frequent given the size of tubes that would be practical. Now the project is getting complex enough that an RO system starts to look attractive! Yep, sure does. Keith Hughes |
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