Towards a better tinyhouse

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Multi effect distillation and vapor compression (evaporation?) for a water recycler

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What if you could just distill the water, in order to remove contaminants? Would clean the water really good, right? Distillation is like the ultimatzorz. Well, it isn’t perfect, but it could certainly do the heavy lifting.

But it requires too much power, right? What most people think of as distillation is single effect distillation. Funnily enough, multi effect distillation doesn’t seem to be mentioned in that article. I should add it later… Anyway The number of “effects” you get out of a distillation process refers to the amount of water you get out per unit of energy compared with single effect distillation. I remember reading that multi stage flash plants typically get 5 to 7 effects (edit: the wikipedia article says otherwise, but it might be a mistake, that it is 23-27whr/kg, 27wh.kg is 23 effects, 626 whr/kg is 1 effect, this needs investigation… if true, solar powered multi stage flash could be practical edit: good news, looks like this is right). Certainly a huge improvement.

However it is possible to get much more. Commercial vapor compression (evaporation? I was thinking of it as distillation, but whatever) units can get a whopping 72 effects. Heck, with that sort of efficiency, to get our required 70 liters a day, you would only need 608 watt hours a day. That could be provided with solar panels without breaking the bank, at $2 a watt for the panels, that’s $507 in my area in winter. You’ll notice there are high temperature units, and low temperature units. The main advantages of a high temp unit is that the interior does not need to be kept under a vacuum, and that the density of the water vapor is much, much higher. This means that the compressor doesn’t have to pump as large a volume of gas (vapor). BTW, just in case it’s not clear, there isn’t supposed to be aby air in the chamber, only water vapor. You can get the air out by bringing the water to a boil, then letting steam gush out a one way valve somewhere, sweeping the air out with it. That is one advantage of a higher temperature system.

You may have heard of a device called the “slingshot” that Dean Kamen, inventor of the segway, invented. This machine is a vapor compression thing. I remember reading somewhere that is is basically a stirling engine directly connected to a compressor, which compresses the water vapor.

Would it be possible to make your own vapor compression or multi effect flash unit (especially if it could get more effects) unit? I think there are 2 main problems:

1. The heat exchanger getting dirty. The heat exchanger can be made big enough that it can work fine when clean. However, if the heat exchagner is in contact with the water, the water boils right at the surface of the exchanger, so it leaves all kinds of gunk behind. At temperatures above 60 deg c or something, scale will form.
I don’t know if you could overcome this by using a teflon surface or something, though.

Flash distillation overcomes the problems with having the boiling liquid in contact with the heat exchanger, however you have a large number of exchangers in different chambers, so if they did get dirty somehow (we are talking about greywater on one side, after all) then they could be a pain to clean, I don’t know. Non stick polymers can help here, though.

The thermal resistance of the heat exchanger is critical to achieving high effects. In fact, that and, for VC the efficiency of the compressor, are pretty much the only things you can change to improve things.

Another idea is to not actually have the heat exchanger in contact with the water. Suppose you had it placed above the water surface, and a fan blowing across it, then over the surface of the water. You might also want to agitate the water somehow. This has the added bonus of the fact that there is no bubbling involved, because bubbling flings droplets of water into the air, and that sucks, because they get can get over to your clean water side.

2. For vapor compression, you need a suitable compressor. You can consult that chart I linked to on vapor density to estimate how much gas it needs to move around per gram of water. The pressure it needs to pump at can be determined by looking at the temperature difference across the heat exchanger, and consulting that table I just linked to on vapor density, which includes the pressures too. So if you were using lowish temperatures, the dirty side is 40 deg c and the clean side is 60 deg c, then you’d have to pump (1/0.0511) M^3 of vapor per kg of water going through the compressor , and across a pressure differential of (149.4 -55.3) torr, or 0.13 atmospheres. A vacuum cleaner mechanism has a static pressure of about 0.2 atmospheres, so it could work at that pressure. Clearly the the higher temperature drop across the heat exchanger, the more powerful a compressor you need.

For higher temps the compressor would have to not mind the higher temperatures, while being surrounded by water, I bet you could find a suitable one, though. An (big) aquarium compressor might actually work, I would have to check into that.

For the chamber, you could use an old pressure cooker. I did that once as a vacuum chamber, and it works pretty good. Use silicone oil around the seal, and your good.

I just sketched a sort of idea, there’s not too much though put into this, but it gives you the general idea. There is a float valve that prevents dirty water from getting into the chamber when it is full, and a valve that prevents brine from escaping when the water level is too low, but it is optional. The manual valve is to constrict the flow of brine getting out, open it wider to increase the flow of brine. You would be sucking the water out with a pump, come to think of it, so you might just control the pump.

There is a sort of s bend in the clean water line, just to try to keep the water vapor in the right side, but I also included an anti vapor valve, because that might not work out very well. The anti vapor valve thing only allows the float valve to open when the water level is high enough, so only liquid water can escape. Actually you might want to put that inside the chamber, but it doesn’t matter much. Then the whole thing is insulated very well, symbolized by the pink rectangle. Then, because the water is at a high temperature, there is a counterflow heat exchanger to get back as much heat as you can. Some needs to escape anyway as waste heat, though(edit partly for my own reference later: could let the waste heat eswcape as steam from a 1 way valve, this takes care of the problem of air getting in too). The main heat exchanger could be a flattened, thin walled metal tube.

You can see the basic design for the multi effect distillation thing on the wikipedia page, just put more stages on to get a reasonable number of effects(edit: might not need more stages very much though it would help a bit). And use a vacuum tube thermal collector. You might only get 7 effects, but the system should be pretty cheap. I think you are limited by the amount of heat the water can store on it’s way to the first chamber(edit,: that is mostly wrong, see above, should be able to get ~23 effects). That might be more practical, the chambers could be mason jars or something. You also might want anti-mist things between the dirty and clean sides, these can be just a piece of stainless steel mesh, pleated into round cartridge, kind of like a water filter cartridge. (edit: you can get steel mesh with 30 micron holes in it, maybe it is possible to gt stuff even smaller too, so it would filter the droplets, or there might be other ways of filtering the droplets out of the gas stream).

These could have a use somewhere, but all things considered I like the MBR more. If you could get a suitable reverse osmosis membrane, that might be more practical, too, and maybe cheaper, given the cost of the solar panels and the compressor (search “reverse osmosis” for a post about reducing costs here). One problem with distilling and reverse osmosis, though, is that you still have to do something with the waste material. You can’t just put it on a garden, though you could dry it and put o the compost heap.

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Written by gregor

October 1, 2010 at 02:44

Posted in Uncategorized

One Response

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  1. Very interesting – you might want to think about Kamen’s patents and reapplication of his original IP here, specifically the peristaltic pump. Imagine using modern materials with their thermal properties in conjunction with their flexibility. A high temperature silicon tube and a peristaltic pump could solve the vapor compression problem and a lot of the build-up issues.

    Anonymous

    November 1, 2011 at 22:56


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