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Low Pressure Water Distillation. Why is it Not Used More?

09/30/2012 10:55 AM

I have been wondering about why boiling and recondensing water to purify it is not done under moderate levels of vacuum in order to drop the effective boiling point and reduce the necessary thermal energy required in return?

I have played around with it a bit some years ago with the concept of distilling water under a moderate vacuum to reduce the boiling point down to the temperature levels a typical phase change type heat pump system would work at with reasonable efficiency. Unfortunately at the time I did not have the necessary tools, materials, and equipment to build a fully functioning system and study it properly.

So whats the fatal flaw in the concept of using a lower boiling point and driving the boiling and condensing phases with an off the shelf type heat pump system to recycle the thermal energy required to drive the process to clean dirty water?

I have the vacuum pumps, water pumps, heat pump, and all the necessary tools and equipment to build a system now but I am just curious as to why this method is not used presently.

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#1

Re: Low pressure water distillation. Why is it not used more?

09/30/2012 11:22 AM

Wouldn't the energy needed to maintain the vacuum offset the savings on thermal input?

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#2

Re: Low pressure water distillation. Why is it not used more?

09/30/2012 12:03 PM

Because its the heat that purifies that water not so much the phase change from a liquid to gaseous states. As far as distillation, keep in mind that while under vacuum it's fractionization can be proportionate to other components in the water, keep in mind what temperature kills bacteria.

But the energy costs is the largest reason, to pull a vacuum. If my memory serves me, If you pull a vacuum of + 29"hg. Water Boils at approx. 115 degrees F. I would think depending on the requirements, reverse osmosis would be more effective.

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#3

Re: Low Pressure Water Distillation. Why is it Not Used More?

09/30/2012 1:12 PM

What happens to water vapor that has been sucked through a vacuum pump? Doesn't it pick up oil vapor?

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#9
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Re: Low Pressure Water Distillation. Why is it Not Used More?

09/30/2012 8:21 PM

Water does not get 'suck through' the vacuum pump. You draw vacuum through the cyclone.

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#10
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Re: Low Pressure Water Distillation. Why is it Not Used More?

09/30/2012 8:23 PM

I understand that.

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#4

Re: Low Pressure Water Distillation. Why is it Not Used More?

09/30/2012 1:38 PM

Marine water makers commonly use vacuum distillation. Triton and Atlas are two mfrs I have operated in the past. The vacuum was better than 29"Hg, but that may depend on how cool the ocean is.

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#5
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Re: Low Pressure Water Distillation. Why is it Not Used More?

09/30/2012 2:04 PM

The ones on offshore drilling rigs use just enough vacuum to boil seawater with engine heat, 200 or so, work great but cleaning a PIA with the heat exchanger inside a pot. When on, no other engine cooling needed, usually so much water it's dumped into the drill water tank after the pot tanks are filled, BUT if little load don't make much.

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#6

Re: Low Pressure Water Distillation. Why is it Not Used More?

09/30/2012 2:38 PM

So it does get used some. I was assuming it had to be in use someplace or if not what its down sides were.

The system I had started experimenting with was based on both the boiler side and condenser side being under equal levels of vacuum. My reasoning on a system like that needing far less power input was due to it having a heat pump system based on common heat pump compressor and freons that are mass produced and cheaper.

The idea was that on all the small home use type distillers they use a load of electrical power to boil off the water then dump most of the heat in the steam condensing process into the air instead of reusing it to heat and boil more water.

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#7
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Re: Low Pressure Water Distillation. Why is it Not Used More?

09/30/2012 2:45 PM

It seems to need a lot of free/waist heat to be profitable, most of the Jack-Up rigs have gone to RO since they only have hotel load if not drilling and there's not enough heat there to keep up with the pot water usage.

I've always been curious why the navy uses RO, would think there would be a lot of heat to use, especially nuclear.

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#41
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Re: Low Pressure Water Distillation. Why is it Not Used More?

10/06/2012 11:57 AM

pump noise showing up on sonar maybe?

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#8

Re: Low Pressure Water Distillation. Why is it Not Used More?

09/30/2012 6:18 PM

In our laboratory in Europe we do a lot of MOLSIV regeneration under vacuum. Most of these balls and pellets require a vacuum. I just don't see how the (steam)vapor can be kept separate from the vacuum path, as Lyn also posted. Our vacuum pumps mix the oil with the vapor but flush it off at the pump outlet. What is coming out there isn't an invitation to consumers. All the power used shows no win win alternative.

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#11

Re: Low Pressure Water Distillation. Why is it Not Used More?

09/30/2012 9:11 PM

I'm not sure but it looks like these people http://bluegoldtech.com/technology/ may be doing what you are asking about in an efficient and cost effective manner.

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#12
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Re: Low Pressure Water Distillation. Why is it Not Used More?

09/30/2012 9:26 PM

Nice find. I'm not in the market now, but if I were back on the crab boat....

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#13

Re: Low Pressure Water Distillation. Why is it Not Used More?

09/30/2012 10:45 PM

They do indeed use vacuum distillations for water purification. They call it multiple effect vacuum distillation with a cascade of equipment as follows. Where you have waste heat it is good, but if you must use oil/gas = costly

theory


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#14

Re: Low Pressure Water Distillation. Why is it Not Used More?

09/30/2012 10:58 PM

The simple fact is that it is usually not worth the complexity & capital cost for something like water- unless it is a very large seawater purification system etc. where they can also use multiple stages to recover heat energy & avoid heat exchanger pinch points. For something like this crystallization (freeze distillation) & RO can often be more efficient.

Where it really shines is for chemical processing. Pressure swing distillation can help break azeotropes (did you say 200 proof??), and there are plenty of chemicals that require a vacuum as they begin decomposing before their boiling point. Vacuum distillation can be an invaluable tool in these situations.

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#32
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Re: Low Pressure Water Distillation. Why is it Not Used More?

10/01/2012 12:04 PM

Its only worth it if a governmental agency is threatening to shut down the process. Then you have to pass along the costs to the customer.

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#15

Re: Low Pressure Water Distillation. Why is it Not Used More?

10/01/2012 12:15 AM

Flash distillation is the main system used for generating bulk potable water from sea water. I have been part of installation team for 8 flash water distillation plants in a Middle East Country each generating 5 Million Gallons of water per day. The vacuum is produced through steam ejectors using steam generated from waste heat from gas turbines. the sea water boils at 70 deg C and is condensed using sea water for cooling.

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#16

Re: Low Pressure Water Distillation. Why is it Not Used More?

10/01/2012 2:04 AM

I agree with Aurizon's answer. The multiple effect evaporation process is widely used to concentrate solutions evaporating the water (the most common solvent). There is a considerble heat saving; 3 efects result in approx 50% thermal energy reduction. The limit on the number of stages or effects is depending on the temperature of the prime heating medium, the temperature of the final condensing heat sink and the cost of equipment including vacuum ejectors or pumps. Multiple effect evaporation is more cost effective for large capacities because of its complexity increases with the number of effects.

Sugar production is the classic example: cane juice is evaporated in 3 to 5 or even 6 effects to obtain a concentrated solution from which sugar crystallizes and is separated by centrifugation. The remaining concentrated solution is molasses.

About the simple concept of just lowering the operation pressure to reduce heat input to evaporate water, there is an important consideration: the sensible heat saving to rise the temperature to the boiling point at vacuum may be offset by the increase in latent heat to boil the water, since it increases as temperature and pressure are lower. Check the steam tables (such as those of Keenan and Keyes).

If feed water is too dirty, some contaminants will evaporate with the water vapor at any pressure conditions and will spoil the distillate. In this case a more complex system is required, say a distillation column to separate the water from the volatile contaminants.

Reverse osmosis (RO) is much simpler and economical for small to medium capacity operations. An economic analysis is required to see if it is more expensive than multiple effect evaporation in a particular case, considering the cost of prime thermal energy for evaporation and electricity to pump the water through the RO membranes.

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#17

Re: Low Pressure Water Distillation. Why is it Not Used More?

10/01/2012 2:19 AM

Another issue is that although water boils at a lower temperature, it actually takes MORE heat to evaporate it.

Heat of vaporization at 0 PSIG (212F) is 974 BTU per pound. Heat of vaporization at 10 in HG (192F) is approx 982 BTU per pound. Heat of vaporization at 20 in HG (161F) is about 1001 BTU per pound.

If you have a ready source of waste heat- great. If it is NEW energy- it will cost more.

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#22
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Re: Low Pressure Water Distillation. Why is it Not Used More?

10/01/2012 10:24 AM

GA, Thanks, telling the Rig Mechanic this is going to be more impressive than why 12K btu is called a TON. Isn't that funny, both trivia "jokes" are thermo mechanical facts most people don't know

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#23
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Re: Low Pressure Water Distillation. Why is it Not Used More?

10/01/2012 10:34 AM

Then explaining the relationship between 12K BTU/hour and one ton of ice is even more fun.

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#24
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Re: Low Pressure Water Distillation. Why is it Not Used More?

10/01/2012 11:02 AM

It is not a problem if you have the entire 24-hour day to explain it.

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#43
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Re: Low Pressure Water Distillation. Why is it Not Used More?

07/09/2014 5:18 PM

I love this topic ! ! I and another HVAC (Heating, Ventilation, Air Conditioning) friend built a low temperature alcohol distillation system. It was based on vacuum evaporation instead of 212 deg boiling. It's "boiler" operated at approximately 100 deg F. and on the Condenser side it was cooled to a few degrees above freezing (35 to 45 deg F) The heat source was also the cooling source. it was a heat pump system that used the heat collected from condenser to heat the boiler. there was also a compressor to circulate the vapors through the system. It supplied compressed hot air to the condenser at about 10 PSI then released that air to the bottom of the boiler and simultaneously maintained a slight vacuum in the air above the beer in the boiler. Since our heat was used and re used many times from one stage of process to the other, we believed that it was efficient, however, we did not do proper study and testing. The output on the first prototype was low (very low) but considering that we had distilled anything at temperatures under 110 deg F I consider it a great success ! ! ! Let me know if anyone is interested in more information about this system. We sank lots of money into it, I would love to share the information with someone who would take up the study it worked, but we ran out of money for research and the price of gasoline fell from $4.00 per gallon to $2.50 or so, and we knew we could not compete with gas prices that low. Best regards Harold

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#44
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Re: Low Pressure Water Distillation. Why is it Not Used More?

09/27/2016 12:22 PM

Is it possible that i can make use of direct heat from the sun to boil the water under low pressure due to vacuum pump?

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#45
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Re: Low Pressure Water Distillation. Why is it Not Used More?

09/27/2016 12:29 PM

A dome, with a vacuum inside has been built with sloping shaded walls at the back for condensation, and it works.

The big problem is that only ~~1200 watts per square meter enters at high noon on a clear day. A water kettle is 1500 watts, so it works but the rate is slow, and the water cools to equlibrium with heat in.

It also works with no vacuum, or a partial one.

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#46
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Re: Low Pressure Water Distillation. Why is it Not Used More?

09/27/2016 12:41 PM

Is 1500 watts that should be used in kettle will decrease since the water inside the space to be heated is in low pressure? Thank you!

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#48
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Re: Low Pressure Water Distillation. Why is it Not Used More?

09/27/2016 12:54 PM

Not by much, if at all. (I'm too lazy to look it up in a steam table.)

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#49
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Re: Low Pressure Water Distillation. Why is it Not Used More?

09/27/2016 1:10 PM

well, if this man wants an unattended solar still, he must pump in the water and pump out the outgassed air

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#50
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Re: Low Pressure Water Distillation. Why is it Not Used More?

09/27/2016 2:03 PM

Water at atmospheric pressure requires 970 BTU per pound to make steam.

Under a 5 inch (Hg) vacuum it requires about 975 BTU per pound and at 15 inches (Hg) it is about 990 BTU per pound- SO as pressure falls, energy usage increases.

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#51
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Re: Low Pressure Water Distillation. Why is it Not Used More?

09/27/2016 2:30 PM

Agreed (see #48), but I should have also noted that with lower pressure, the water will need less sensible heat to raise it from ambient temperature to boiling temperature. Thus Metom could gain there.

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#52
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Re: Low Pressure Water Distillation. Why is it Not Used More?

09/27/2016 9:36 PM

Assuming a 60F starting point-

Atmospheric boiling requires (212-60) or 152 BTU plus 960 BTU for a total of 1112 BTUs.

15 inches (Hg) is about 180F so it requires (180-60) or 120 BTU plus 990 BTU for a total of 1110 BTUs SO you can make low pressure steam with 0.18% (2/1110) less energy input.

THAT makes all the difference.

Of course, if you start with 40F water, the added energy if atmospheric is 0.177% more.

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#53
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Re: Low Pressure Water Distillation. Why is it Not Used More?

09/27/2016 10:04 PM

Since this "conversation" got started about 4 years ago, I and my company have developed a system we call "SaveYore" that can reclaim ANY "dirty water"- frac water, sea water OR sanitary waste water- into distilled water by using the waste heat from a CoGen (CHP) system.

It takes the incoming "dirty" water and uses the steam produced to pre-heat it, then uses the waste heat to boil it in a vacuum. The steam goes through a steam to water HXU where the incoming water is heated and SOME of the steam is condensed then the rest of the steam goes to an air-cooled condenser mounted above so that the distilled water falling from the condenser creates/maintains the vacuum as it falls through a trap on its way to the receiver tank.

The sanitary waste is turned into a thick slurry that is fed to a digester to make methane to help support the generator fueling. Frac water is also made into a slurry which is used as part of the "fresh" charge.

The waste heat also creates dry sea salt and turns the solids from the digester discharge slurry into dry fertilizer so ALL of the water is reclaimed.

The system produces 110% of the "raw" input as output because the CoGen system actually produces a moderate amount of near-potable water that is condensed exhaust water vapor (which is also "reclaimed").

Another "neat" use is food processing sites where the waste water gets additionally "polluted" by food particle "fines" that trigger high VOC discharges and require added processing by the water treatment agency- which triggers surcharges on the waste water discharge. THIS system can eliminate ANY sewer charges AND offset significant FRESH water purchases. The distilled water can be used for ANY activity, even "potable water" so long as it is treated with Chlorine like any other potable water system.

ALL of this is happening WHILE the system is PRODUCING POWER, NOT USING IT.

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#54
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Re: Low Pressure Water Distillation. Why is it Not Used More?

09/27/2016 10:10 PM

The "SaveYore" post was accidentally posted as "OFF TOPIC". It is obviously totally in line with all prior postings.

Sorry

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#55
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Re: Low Pressure Water Distillation. Why is it Not Used More?

10/27/2017 12:49 AM

I'm extremely interested In your project I would like to know more... I realize your post was in 2014 Lol

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#18

Re: Low Pressure Water Distillation. Why is it Not Used More?

10/01/2012 3:28 AM

It is done, daily, on ships.

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#47
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Re: Low Pressure Water Distillation. Why is it Not Used More?

09/27/2016 12:43 PM

Yes, commonly with waste steam. Solar is also used

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#19

Re: Low Pressure Water Distillation. Why is it Not Used More?

10/01/2012 3:58 AM
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#20

Re: Low Pressure Water Distillation. Why is it Not Used More?

10/01/2012 6:22 AM

Guys,

One issue that always surrounds desalination is the impact of extracting "pure water" that our civilization has on the planet.

Simply put all or most desalination plants will put a higher level of salt/brine back into the environment and it has to be assumed that this is non reversible. The additional salt either ends up back in the ocean or on land. Either way this is yet another poison that we inflict upon our living space.

Has anyone access to the numbers? I used to but this has vanished with my filing system!

The concept that I am addressing here is that any salt water desalination plant will take in a number of litres a day of saltwater, process that and push the waste product, primarily heavily concentrated salt water aka brine back into the sea.

The result is a slow but steady increase in the salt water content of the ocean/sea surrounding each such plant and in certain areas such as the Gulf that can have a considerable impact on the surounding environment - further to travel for fish each day is one of the smaller impacts.

Has anyone got real numbers for individual regions and better still has anyone got any thoughts as to how thios impact can be countered.

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#21
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Re: Low Pressure Water Distillation. Why is it Not Used More?

10/01/2012 9:40 AM

The salt concentration rise in the sea may be true in some locations with low currents or an almost enclosed geography, such as the Persian Gulf, the Black Sea, etc. and in the vicinity of the brine discharge.

But in general, the effect does not exist since all the "pure" water once used returns to the sea. And Mother Nature distills huge amounts of water every hour; we see it as clouds and rain/snow.

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#25
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Re: Low Pressure Water Distillation. Why is it Not Used More?

10/01/2012 11:11 AM

Oscar,

That is why I asked has anyone got access to the actual numbers - that is what makes my argument succeed or fail.

We cannot debate without facts - this argument was presented to me in a discusion about a large Mediterranean project many years ago. The leader of these arguments has since disappeared - old age? But he was very persuasive at the time.

Maybe we need access to the global weather figures?

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#28
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Re: Low Pressure Water Distillation. Why is it Not Used More?

10/01/2012 11:51 AM

I have found "THE NUMBERS". I googled "historic ocean salinity values".

They are in a compilation report prepared by NOAA in 1994 where they went to a wide variety of data files compiled over several years and analyzed that data for salinity, temperatures and oxygen content at depths from 10 meters to 5500 meters.

It is in

ftp://ftp.nodc.noaa.gov/pub/data.nodc/woa/PUBLICATIONS/qc94tso.pdf

Salinity values start on page 45 (sheet 52 of 72) and show very narrow bands between high and low observed values for nearly all oceans at depths below about 600 to 800 meters.

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#29
In reply to #25

Re: Low Pressure Water Distillation. Why is it Not Used More?

10/01/2012 11:52 AM

You are right. The numbers are essential to make an analysis in a certain region; but with the complexity and extent of any sea region, a study requires a lot of information from extensive studies and dedicated teams of highly qualified professionals.

A study of some areas of almost closed seas is feasible and data should exist; then defining a mathematical model to simulate the effects of desalination plants and the total balance of water and salts including human and natural causes will take time and money. The Black, Baltic, Adriatic seas and the Persian Gulf may be done; but the Mediterranean is another order of magnitude.

But for the whole ocean the problem does not exist since all the water evaporated by natural and human proceses will go back to the ocean. There is no way to increase the overall salinity unless the remaining salt reserves existing on land are washed down to the sea.

An additional problem is the global warming that causes melting of glaciers and large ice fields like Greenland and Antarctica. A dilution of sea salinity is being observed now in some areas.

In sum, numbers are essential but they shall be reliable.

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#34
In reply to #25

Re: Low Pressure Water Distillation. Why is it Not Used More?

10/01/2012 12:12 PM

You purchase and download Water And Steam Program, (WASP) program.

I believe I have an excel program for this, PM me, and I'll send it to you.

This is what I had done when I designed my Brine Concentrator (Still). Even though I was not making potable water, I was appling it to concentrate the brine from my earlier post. But the numbers are there, and realtively easy.

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#36
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Re: Low Pressure Water Distillation. Why is it Not Used More?

10/01/2012 12:28 PM

THANKS, PHOENIX.

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#26
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Re: Low Pressure Water Distillation. Why is it Not Used More?

10/01/2012 11:25 AM

WALOOB. It blends with the water that has been removed from it via water courses following sewage treatment.

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#27
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Re: Low Pressure Water Distillation. Why is it Not Used More?

10/01/2012 11:35 AM

PWSlack

What is WALOOB?

Sleepy

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#30
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Re: Low Pressure Water Distillation. Why is it Not Used More?

10/01/2012 11:54 AM

Ask Del.

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#38
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Re: Low Pressure Water Distillation. Why is it Not Used More?

10/03/2012 11:55 PM

Are you serious? Just as a little thought. The salt was already in the ocean. So we take it out and put it back in. There is no gain on that one.

The water we take out will end up either in the ocean after we use it either via canalisation or in the rain.

I think the amount of water evaporated over the ocean is times higher than what little desalination plants are taking out.

Not an issue in the whole!

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#31

Re: Low Pressure Water Distillation. Why is it Not Used More?

10/01/2012 12:03 PM

I had actually Designed what I call a Brine Condensing unit........Company didn't like me calling it a still.

I designed it for a California Dairy for producing Cheese.

Basically to condense water off the Brine to keep the Salt Concentration levels. It pulled about 2 GPM out.

Because due to process in moulding 20 pound cheese blocks, ice water was added to the brining lake. hence deluting the brine concentration, they then had to dispose of the excise deluted brine (spreading it on the fields) because the environmental agencies were really buckling down on the dairy for that practice.

They then added salt to bring the salt concentration back to the brine.

I had calculated that 72 tons of cooling was required to cool the brine back down for the process. Our project manager which also was the salesman (you know where this is going) that sold them the unit, said thats not a problem they have plenty of cooling capacity...... I pressed him for a hard number, never got it.

My concerns push me to contact the plant manager which his response was that they have a 100 ton chiller and they use about 25-30 tons for production.

He was wrong, they used approx. 62 tons of cooling for production.

Bringing the condenser on line, Brine had to kept at 28 Degree F., when the condenser was online, it did not take long for it to creep to 70 degrees.

In a cheese processing plant, normally there never is a probally with the capacity to heat, the problem usually is cooling capacity.

Keep this in mind!

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#37
In reply to #31

Re: Low Pressure Water Distillation. Why is it Not Used More?

10/01/2012 2:11 PM

Good story !

Unfortunately similar to others I have heard.

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#33

Re: Low Pressure Water Distillation. Why is it Not Used More?

10/01/2012 12:09 PM

Mist entrainment is another problem. Bubbles pop and spray distilland mist, which is entrained with the vapor stream, so the distillate is contaminated. For suppressing mist entrainment in vacuum distillation, see http://www.freepatentsonline.com/5534118.pdf and for demisting see http://www.freepatentsonline.com/5688377.pdf

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#35

Re: Low Pressure Water Distillation. Why is it Not Used More?

10/01/2012 12:25 PM

Wow this is proving more educational than I expected!

My thoughts with using the heat pump system wast to cut down considerably on the total energy required to distill the water by simply recirculating the heat energy in the unit as a whole so once it was up to operating temp the heat pump would keep the two temperatures between the evaporator side of the distiller and the condensing side stable with the hopes that the energy input driving the heat pump itself would be sufficient to keep up with parasitic thermal losses coming from the hot brine being extracted and overall system thermal loses.

What I am after is to design a system that can clean up normal municipal water and well water not necessarily high salinity sea water or heavily contaminated/dirty water.

Basically I just like to play around and experiment with odd ideas now and then and making my well water cleaner by using a less conventional distillation method seemed like a fun curiosity to work on!

That and it sort of justifies all of my collecting odd junk and mechanical bits en pieces for supposed future experiments.

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#39

Re: Low Pressure Water Distillation. Why is it Not Used More?

10/05/2012 2:23 PM

In nature, mass solar distillation is taking place daily. Evaporation & Condensation.

The very formation of water vapours separated from saline sea or surface water is the basic step of desalination.

Any thoughts on this daily phenomenon?

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#40

Re: Low Pressure Water Distillation. Why is it Not Used More?

10/06/2012 4:00 AM

The process of vacuum distillation to purify water has been been used since Noah had it on the ark!!!! Not literally speaking of course.

At sea, ships used this process of not only making water for domestic use, but more importantly for the making of feed water for the boilers and of course this had to be of a very high purity as far as salts were concerned as it would cause severe and unwanted corrosion. Evaporators as they were called were labour and maintenance intensive and back in those days there was no automation involved, they always had to have a watchkeeper 24/7 because they were so temperamental. A vessel that the Academy where I work had a reverse osmosis system and I can't make up my mind which was worse.

From what I can recall the only place where we would shut down the evaporators was when we entered Hong Kong Harbour, due to the danger that the water boiling point would be insufficient to kill a lot of the bugs there in the harbour.

A big problem with large desal plants apart from maintenance and labour costs is that they are energy hungry, in Perth, Western Australia, they did it right, they built a solar power station next to the desal plant to satisfy its energy hunger.

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#42

Re: Low Pressure Water Distillation. Why is it Not Used More?

06/14/2013 9:24 PM

The energy required to maintain the vacuum may outweigh the perceived gains otherwise. Entropy, unfortunately, is always one step ahead of enthalpy!

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