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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 |
Potable Water - The Third Way.
Mark Borgerson wrote:
In article , says... Mark Borgerson wrote: In article , says... snip snip 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. I see the problem. I am assuming that you completely fill a 40 foot tube with water using a pump capable of providing about 16-20 PSIG. That fills the tube completely with water--at which point you close the tube (with a one-way valve). Uhmm, a manual valve is a manual valve. A "one-way" valve is a checkvalve, and you wouldn't need to close it. When you release the pressure at the bottom end, the water falls to the point where the weight of the water column is one atm (about 14.7PSIA) minus the vapor pressure of water at 20deg C. The vapor pressure of water at 20C is about 17.5mmHg, or about 2.3% of the 760mmHg standard atmosphere. Since a mercury has a density 13.6, the column of water will be 13.6 * (760- 17.6)mm high. That's 10.1m high, or about 33.12 feet high. In a 40-foot tube, that would leave about 7 feet of water vapor at the top of the tube and 33 feet of water below the vapor. Ahh, no. See below... 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. What is the 1 liter to which you refer? It is an example, for illustration purposes. It's the headspace (i.e. the amount of volume *not* filled with water, prior to closing the valve and letting the water columns 'fall'). The point is, whatever your starting headspace volume is, to get anywhere near a vacuum, the *VOLUME* of the headspace must increase 100 fold. That is the relevance of the ideal gas law. If you start with a 100ml headspace, then to get a decent vacuum, the water columns have to drop to a point where the headspace is 10L. *AT THAT POINT* you have sufficient vacuum to support a water column of around 30'. But the columns have dropped significantly to achieve that vacuum, and thus the columns must be much higher, as must the initial water column height. The *only* way the headspace volume increases is if the water columns drop significantly, and the only way significant vacuum is created is if the sealed volume increases tremendously. This is not a closed system---the tube is open to a reservoir at atmospheric pressure at the bottom. I'm assuming that you start with a head space (or initial volume) of zero. You then simply have to evaporate enough water to fill the top of the tube with water vapor to the point where vapor pressure + water weight = 1ATM. 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. Well, not much higher----only about 17.5 mmHG higher. But that IS a lot higher than zero! ;-) No, a lot higher. You're confusing vapor pressure with the "steam" pressure during distillation. Huge difference. Vapor pressure is the countervailing force (fighting condensation as it were) on the FRESH water side of the system (and the seawater side). Vapor pressure in both columns will be about the same, so you have to boil the seawater column to get any significant vapor transfer. This results in *much* higher pressure, which lowers the columns and increases the vapor path and.... 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. AHA!, you're assuming a much higher operating temperature than me. Yes, because you're mistaking the amount of "vacuum" you'll have available when the system reaches equilibrium. I was assuming something on the order of 20 to 25C. Then you are assuming an almost perfect vacuum, which can't happen since the boiling Must significantly raise the headspace pressure. You're going to have to add to your energy budget the heat necessary to raise the water temperature from 20C to 60C, then. If the equilibrium pressure is really 1/3ATM, then there will be about 20 feet of water in the 40-foot tube and 20 feet of vapor. If you're going to work at those temperatures and pressures, you probably need only a 22-foot tube. 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. Keith Hughes |
Potable Water - The Third Way.
Boiling Point Elevation
The boiling point of a solution is higher than that of the pure solvent. Accordingly, the use of a solution, rather than a pure liquid, in antifreeze serves to keep the mixture from boiling in a hot automobile engine. As with freezing point depression, the effect depends on the number of solute particles present in a given amount of solvent, but not the identity of those particles. If 10 grams (0.35 ounces) of sodium chloride are dissolved in 100 grams (3.5 ounces) of water, the boiling point of the solution is 101.7°C (215.1°F; which is 1.7°C (3.1°F) higher than the boiling point of pure water). The formula used to calculate the change in boiling point ( Tb) relative to the pure solvent is similar to that used for freezing point depression: Tb = i Kb m, where Kb is the boiling point elevation constant for the solvent (0.52°C·kg/mol for water), and m and i have the same meanings as in the freezing point depression formula. Note that Tb represents an increase in the boiling point, whereas Tf represents a decrease in the freezing point. As with the freezing point depression formula, this one is most accurate at low solute concentrations. From: http://www.chemistryexplained.com/Ce...roperties.html |
Potable Water - The Third Way.
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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 |
Potable Water - The Third Way.
In article ,
says... 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. The head space is generated by the evaporation (or boiling) of some of the water in a column. It's exactly the same principle that you get it you fill a closed tube full of mercury and then invert it, placing the end in a reservoir of mercury. (We call these things barometers.) You start with no head space, but when you invert it, VOILA! head space appears as the mercury sinks to a level where the weight of the mercury equals the atmospheric pressure. You get a much better vacuum with mercury, since it has a much lower vapor pressure at room temperature. A column of water will behave the same way. The column just has to be much taller. Some of the historical references on water barometers mention that, despite precautions, the water in the barometer eventually got contaminated with dissolved gases and they lost their accuracy. 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.\ I agree with that part---except for the oscillation part. I think the processes are slow enough and the thermal and physical masses are high enough that the oscillations will be damped out and you will see a slow change to equilibrium with little or no overshoot. Mark Borgerson |
Potable Water - The Third Way.
N:dlzc D:aol T:com (dlzc) brought forth on stone tablets:
Dear Richard Casady: "Richard Casady" wrote in message ... On Sat, 29 Sep 2007 19:44:35 -0700, Keith Hughes wrote: Sounds like perpetual motion to me, but I'm having a hard time envisioning what you're describing above. Of course you are, since it is basically nonsense. No mention of where the energy comes from. Links were provided. "waste heat" (from what process?) and / or "solar heat" have been cited so far. All the vacuum does is move boiling temperature closer to ambient. Making more common materials suitable for this application. David A. Smith .... and much more importantly, making the process viable with far lower quality heat. bob s/v Eolian Seattle |
Potable Water - The Third Way.
On Mon, 08 Oct 2007 10:13:17 -0700, Keith Hughes
wrote: ...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. .... A solid liquid loop will not separate into two separate columns. .... Keith Hughes Perhaps it would be better for you to check what is the maximal rise (head) of a syphon. Can you guess what it might be? Brian Whatcott Altus OK |
Potable Water - The Third Way.
On Mon, 08 Oct 2007 05:33:39 -0700, Keith
wrote: Boiling Point Elevation The boiling point of a solution is higher than that of the pure solvent. Accordingly, the use of a solution, rather than a pure liquid, in antifreeze serves to keep the mixture from boiling in a hot automobile engine..... From: http://www.chemistryexplained.com/Ce...roperties.html Actually, no. Ethylene glycol in its pure liquid state boils near 200 degC http://www.dow.com/ethyleneglycol/about/properties.htm It is usually cut to 50% dilution for use as an antifreeze. Brian Whatcott Altus OK |
Potable Water - The Third Way.
On 2007-10-08 18:17:58 -0400, Mark Borgerson said:
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. I agree with that part---except for the oscillation part. I think the processes are slow enough and the thermal and physical masses are high enough that the oscillations will be damped out and you will see a slow change to equilibrium with little or no overshoot. Though I consider this whole discussion impractical, I haven't seen anyone mention that the fresh-water side will be drawn down fairly regularly. And, of course, the sea water side will be replenished from time to time. Suck hard enough on the fresh-water side and you get even better "vacuum" at the top. (Dissolved gasses are likely to be a problem, though.) Cool the fresh-water side and water vapor will condense there -- the whole point of the exercise. Thinking only momentarily on a problem that I have little interest in... if the fresh-water side is evacuated to the point that the salt-water side is slightly below the top, every once in a while (or perhaps often), the fresh-water side will be empty and only the previously-dissolved gasses evacuated. The required evacuation pumps and one-way valves sound like the problem at the moment. -- Jere Lull Tanzer 28 #4 out of Tolchester, MD Xan's new pages: http://web.mac.com/jerelull/iWeb/Xan/ Our BVI pages: http://homepage.mac.com/jerelull/BVI/ |
Potable Water - The Third Way.
Brian Whatcott wrote:
On Mon, 08 Oct 2007 10:13:17 -0700, Keith Hughes wrote: ...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. ... A solid liquid loop will not separate into two separate columns. ... Keith Hughes Perhaps it would be better for you to check what is the maximal rise (head) of a syphon. Can you guess what it might be? Uhmmm, maybe you should follow the thread. If you *pump* the water up the columns, evacuating the headspace as you go, until the columns meet, you can siphon pretty much any height you want. Until outgassing creates a headspace. If the bases of the columns are at differing heights, you have a siphon until that happens. Keith Hughes |
Potable Water - The Third Way.
Mark Borgerson wrote:
In article , says... 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. The head space is generated by the evaporation (or boiling) of some of the water in a column. It's exactly the same principle that you get it you fill a closed tube full of mercury and then invert it, placing the end in a reservoir of mercury. You seem to be forgetting that the whole purpose is to Purify/desalinate the water. No initial headspace = single process stream = contamination on the distillate side. (We call these things barometers.) Except when we call them Mcleod gauges... You start with no head space, but when you invert it, VOILA! head space appears as the mercury sinks to a level where the weight of the mercury equals the atmospheric pressure. You get a much better vacuum with mercury, since it has a much lower vapor pressure at room temperature. A column of water will behave the same way. The column just has to be much taller. True, but you need to keep the context - water purification. The contamination control features are as crucial to the operational constraints as are the physical parameters. Thus, you have to *Start* with headspace. Sure, you could purge the freshwater side until the contaminants are removed, but by then most, if not all, of your production will be wasted. Some of the historical references on water barometers mention that, despite precautions, the water in the barometer eventually got contaminated with dissolved gases and they lost their accuracy. Yes, you can only deaerate so far prior to filling. Personally, I've never seen an absolute pressure water barometer. IME they are primarily used in an inverted u-tube configuration for DP measurements. BTW, mercury barometers suffer the same fate, primarily through oxidation of the mercury, changing the density. Just look at that almost black film layer on any old barometer. I agree with that part---except for the oscillation part. I think the processes are slow enough and the thermal and physical masses are high enough that the oscillations will be damped out and you will see a slow change to equilibrium with little or no overshoot. You may be right, but I doubt it. Unless you control the temperature versus pressure relationship, which is virtually impossible with any passive heating process, then I'd expect self quenching would result in an oscillating system. Keith Hughes |
Potable Water - The Third Way.
On Mon, 08 Oct 2007 17:36:59 -0700, Keith Hughes
wrote: ... If you *pump* the water up the columns, evacuating the headspace as you go, until the columns meet, you can siphon pretty much any height you want. ... Keith Hughes No, you cannot syphon to any height. Yes, you can pump to any height, within reason. There is a difference. If you pump to 40 feet head, quite filling an inverted u-tube, for instance, when the pump is halted, the water falls until a particular maximum height is no longer exceeded, if the foot of the u-tube is opened to water whose surface is at atmospheric pressure.. Brian W |
Potable Water - The Third Way.
Brian Whatcott wrote:
On Mon, 08 Oct 2007 05:33:39 -0700, Keith wrote: Boiling Point Elevation The boiling point of a solution is higher than that of the pure solvent. Accordingly, the use of a solution, rather than a pure liquid, in antifreeze serves to keep the mixture from boiling in a hot automobile engine..... From: http://www.chemistryexplained.com/Ce...roperties.html Actually, no. Ethylene glycol in its pure liquid state boils near 200 degC http://www.dow.com/ethyleneglycol/about/properties.htm It is usually cut to 50% dilution for use as an antifreeze. Brian Whatcott Altus OK Since he used the term "solvent", the assumption, at least on my end, is that he's talking about a solvent/solute system, not a solution of miscible liquids. Keith Hughes |
Potable Water - The Third Way.
Jere Lull wrote:
On 2007-10-08 18:17:58 -0400, Mark Borgerson said: snip Though I consider this whole discussion impractical, I don't know that I'd call the *discussion* impractical; the device certainly. Kind of the point of the discussion. I haven't seen anyone mention that the fresh-water side will be drawn down fairly regularly. And, of course, the sea water side will be replenished from time to time. Suck hard enough on the fresh-water side and you get even better "vacuum" at the top. (Dissolved gasses are likely to be a problem, though.) Cool the fresh-water side and water vapor will condense there -- the whole point of the exercise. Thinking only momentarily on a problem that I have little interest in... if the fresh-water side is evacuated to the point that the salt-water side is slightly below the top, every once in a while (or perhaps often), the fresh-water side will be empty and only the previously-dissolved gasses evacuated. The required evacuation pumps and one-way valves sound like the problem at the moment. The whole exercise was to get a passive system. If you're going to add a vacuum pump, then you just provide continuous evacuation on the freshwater side, using a demister that drains into the freshwater pool, to separate the water vapor from the non-condensables. But if you accept the need for a pump, why use this rather byzantine approach at all? Keith Hughes |
Potable Water - The Third Way.
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Potable Water - The Third Way.
In article , betwys1
@sbcglobal.net says... On Mon, 08 Oct 2007 05:33:39 -0700, Keith wrote: Boiling Point Elevation The boiling point of a solution is higher than that of the pure solvent. Accordingly, the use of a solution, rather than a pure liquid, in antifreeze serves to keep the mixture from boiling in a hot automobile engine..... From: http://www.chemistryexplained.com/Ce...roperties.html Actually, no. Ethylene glycol in its pure liquid state boils near 200 degC http://www.dow.com/ethyleneglycol/about/properties.htm It is usually cut to 50% dilution for use as an antifreeze. True---when mixing liquids, the boiling point is somewhere between the boiling points of the two. Radiators and cooling systems are pressurized so that the system can have an elevated boiling point. Mark Borgerson |
Potable Water - The Third Way.
In article ,
says... Mark Borgerson wrote: In article , says... 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. The head space is generated by the evaporation (or boiling) of some of the water in a column. It's exactly the same principle that you get it you fill a closed tube full of mercury and then invert it, placing the end in a reservoir of mercury. You seem to be forgetting that the whole purpose is to Purify/desalinate the water. No initial headspace = single process stream = contamination on the distillate side. (We call these things barometers.) Except when we call them Mcleod gauges... You start with no head space, but when you invert it, VOILA! head space appears as the mercury sinks to a level where the weight of the mercury equals the atmospheric pressure. You get a much better vacuum with mercury, since it has a much lower vapor pressure at room temperature. A column of water will behave the same way. The column just has to be much taller. True, but you need to keep the context - water purification. The contamination control features are as crucial to the operational constraints as are the physical parameters. Thus, you have to *Start* with headspace. Sure, you could purge the freshwater side until the contaminants are removed, but by then most, if not all, of your production will be wasted. With the proper placement of the check valves, I think you could start with the initial boiling happening in the freshwater side--- after all it is going to boil at a lower temperature. The procedure might look like this: 1 Pump both fresh and salt water to near the top. 2. Shut offf the salt water side pump, but keep the tube closed at the bottom. 3. Pump a bit more fresh water into the tube---where it overflows to the sal****er side, displacing the rest of the air out the check valve. You now have no air in the tube and a small layer of fresh water on top of the salt water. 4 Release the pressure at the bottom, and the fresh water at the top will boil and create your head space with little or no contamination of the freshwater side. 5 Apply your heat differential and remove distilled fresh water as it overflows the reservoir at the bottom. This should work until the dissolved gas problem lengthens the vapor path to the point where you have to start over at step 1. Some of the historical references on water barometers mention that, despite precautions, the water in the barometer eventually got contaminated with dissolved gases and they lost their accuracy. Yes, you can only deaerate so far prior to filling. Personally, I've never seen an absolute pressure water barometer. IME they are primarily used in an inverted u-tube configuration for DP measurements. BTW, mercury barometers suffer the same fate, primarily through oxidation of the mercury, changing the density. Just look at that almost black film layer on any old barometer. That is mostly due to contaminants trapped in the glass and impurities in the mercury. Production barometers didn't use glass that was heated with a vacuum to remove contaminants. I agree with that part---except for the oscillation part. I think the processes are slow enough and the thermal and physical masses are high enough that the oscillations will be damped out and you will see a slow change to equilibrium with little or no overshoot. You may be right, but I doubt it. Unless you control the temperature versus pressure relationship, which is virtually impossible with any passive heating process, then I'd expect self quenching would result in an oscillating system. What do you mean by "self-quenching"? Mark Borgerson |
Potable Water - The Third Way.
On Mon, 8 Oct 2007 18:22:14 -0700, Mark Borgerson
wrote: .... If you pump both the fresh and salt water to the top of the U-Tube, then switch from the pumps to the reservoirs at the bottom, you won't get a siphon. The boiling of water at the top will break the siphon action. Mark Borgerson I am regretting this already. But If I repeat this little test, pumping mercury up an inverted U-tube to 35 inches, when I stop the pump and open the tubes to a mercury pool, the mercury levels in the two tubes drop to a 29.92 inch column each side. The mercury does not boil. 29.92 inches is 760 mm of mercury, by the way. So boiling is not essential to breaking a syphon. Excess height is all that is needed. Brian W |
Potable Water - The Third Way.
Mark Borgerson wrote:
SNIP With the proper placement of the check valves, I think you could start with the initial boiling happening in the freshwater side--- after all it is going to boil at a lower temperature. Well, except that 1) unless you fill the entire apparatus with fresh water (ignoring for a moment how much fresh water that might take, and how much system capacity is lost in re-distilling the fresh water), you haven't eliminated the carryover contamination issue, since you still have a contiguous water stream, and 2) the freshwater side of the system is configured for *cooling* and the seawater for heating (passive system remember), so boiling will always be initiated on the seawater side. The procedure might look like this: 1 Pump both fresh and salt water to near the top. 2. Shut offf the salt water side pump, but keep the tube closed at the bottom. 3. Pump a bit more fresh water into the tube---where it overflows to the sal****er side, displacing the rest of the air out the check valve. And where is the barrier layer that keeps salt from moving into the freshwater? You now have no air in the tube and a small layer of fresh water on top of the salt water. But it won't stay that way. As soon as you begin to heat the seawater, you'll almost certainly have seawater rising into the fresh (do to the density change with heating) before you get boiling going on. 4 Release the pressure at the bottom, and the fresh water at the top will boil and create your head space with little or no contamination of the freshwater side. Yeah, but "little" is not the goal. And you'd have to quantify that "little" empirically, since there are many factors that contribute to the process. 5 Apply your heat differential and remove distilled fresh water as it overflows the reservoir at the bottom. This should work until the dissolved gas problem lengthens the vapor path to the point where you have to start over at step 1. Again, the design complexity involved in being able to heat the fresh side to initiate the boiling there *first*, and then switching to cooling mode when there is sufficient column separation puts paid to any thoughts of this being a simple system. And then, you have a very complex, and Horribly inefficient system. There are lots of ways that you could make the system work, but why? The *only* feature this concept has going for it to start with is simplicity, and basically a passive (save for some human work input) system. Keith Hughes |
Potable Water - The Third Way.
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Potable Water - The Third Way.
In article , betwys1
@sbcglobal.net says... On Mon, 8 Oct 2007 18:22:14 -0700, Mark Borgerson wrote: ... If you pump both the fresh and salt water to the top of the U-Tube, then switch from the pumps to the reservoirs at the bottom, you won't get a siphon. The boiling of water at the top will break the siphon action. Mark Borgerson I am regretting this already. But If I repeat this little test, pumping mercury up an inverted U-tube to 35 inches, when I stop the pump and open the tubes to a mercury pool, the mercury levels in the two tubes drop to a 29.92 inch column each side. The mercury does not boil. 29.92 inches is 760 mm of mercury, by the way. So boiling is not essential to breaking a syphon. Excess height is all that is needed. That is correct. The difference is that water will boil at room temperature because the vapor pressure is much higher. Mercury will not---or at least the evaporation to produce the equilibrium vapor pressure will not require visible boiling. Mark Borgerson |
Potable Water - The Third Way.
On Mon, 08 Oct 2007 18:08:51 -0700, Keith Hughes
wrote: he whole exercise was to get a passive system. If you're going to add a vacuum pump, then you just provide continuous evacuation on the freshwater side, using a demister that drains into the freshwater pool, to separate the water vapor from the non-condensables. But if you accept the need for a pump, why use this rather byzantine approach at all? The whole idea here seems ridiculous. This is nothing but a solar still. Reducing the boiling point is not necessary. All the energy absorbed, or nearly, will evaporate water. The limiting factor is the energy input. There is no benefit to making a modest capacity still thirty feet tall, and skinny. Make it short and fat and save material and weight. Did anyone mention weight aloft and windage? The hot side of the skinny job will be well cooled by the surrounding air. Casady |
Potable Water - The Third Way.
On Mon, 08 Oct 2007 23:05:14 -0700, Keith Hughes
wrote: puts paid to any thoughts of this being a simple system. And then, you have a very complex, and Horribly inefficient system. Gee, I thought I was the only one to notice that. No sarcasm intended. Casady |
Potable Water - The Third Way.
Dear Richard Casady:
"Richard Casady" wrote in message ... On Mon, 08 Oct 2007 23:05:14 -0700, Keith Hughes wrote: puts paid to any thoughts of this being a simple system. And then, you have a very complex, and Horribly inefficient system. Gee, I thought I was the only one to notice that. No sarcasm intended. Actually, the dicussion has been capable of complexifying even a paper clip. David A. Smith |
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