I did quite a few experiments over the last few months, trying to find the best way to separate absorbed water from Calcium Chloride brine, as shown here.
The way to do this is to apply high heat to the brine, in order to drive off the water. As it turns out, it takes quite a bit more energy to drive off the water than to just evaporate pure water. The reason is because of the high affinity and molecular attraction that Calcium Chloride has for water. I did an experiment where I put equal amounts of pure water and Calcium Chloride brine inside equivalent containers and placed them outside in the sun. The pure water evaporated quickly but the brine hardly evaporated at all. It is not enough to just leave it outside in the sun. The energy of evaporation is insufficient.
So what I did was use the same stainless steel tray that I used in the previous post, and put the brine into it, around 1 cm deep. I then placed panes of glass on top to create a sort of solar collector, or “greenhouse”. Alternatively, you can probably use plexiglass instead as it’s not fragile.
I placed small bricks underneath the glass to create a small gap (maybe 0.5-1 cm) between the edges of the tray and the glass to allow the evaporated water to escape. I also placed a sheet of styrofoam underneath the tray to insulate it underneath, allowing less heat to escape from the bottom. See Figure 1. This allows the interior of the tray to get even hotter.
Now, stainless steel is a very good absorber of solar radiation, meaning it gets hot quickly under the sun. Other types of metal, like aluminum, don’t work nearly as well. Now, if the tray you are using is a poor absorber of solar radiation (i.e. it doesn’t get hot) then you can try painting it black, using a paint that is waterproof and can resist the corrosive effects of salt. And it’s a good idea to keep the level of brine in the tray shallow (around 1 cm deep at most). If it’s too deep it will filter out much of the intensity of the solar radiation that reaches the tray, which is key to obtaining a high temperature.
With this set up I had, the heat inside the tray rose to 70-75 degrees Celsius, provided it was sunny with no clouds. And given the large surface area of the tray (60 cm x 60 cm) the rate of evaporation achieved was decent. So it’s a good idea to use a large tray if you have it, preferably made of metal and not plastic. If you want to decant the brine into a separate container between each use you probably shouldn’t use a tray that’s too large. Fortunately, for my size of tray I was able to tilt and decant it under my own power without spilling.
After several weeks of doing this, off and on, during sunny days (in areas of my backyard with no shade), I evaporated a significant percentage of the water, leaving behind a more concentrated brine. Now, once you get the percentage of water down to a certain point, you will start to notice something interesting happening. Crystals will form once the brine cools. What’s happening is that at lower temperature the solubility of salt in the water decreases, and it starts to come out of solution in the form of crystals. And you get some interesting shapes. They look a bit like the structures you’d see in the Fortress of Solitude! See Figure 2.
The crystals tend to form and settle at the bottom, leaving the liquid portion at the top. You can pour this liquid portion back into the tray for further evaporation. After doing this for a while, the amount of crystals formed grow until there’s little liquid left (Figure 3). At this point you’ve basically gotten as far as you can with the sun’s evaporation. At this point the water is bonded very strongly with the salt and you need a greater source of heat to drive off the remaining water. You can use a solar concentrator, like a parabolic reflector, as it concentrates the solar energy to a point, to give you higher heat than you would with a “greenhouse” based collector design. To see examples have a look at http://solarcooking.org/plans/default.htm. The drawback is that you have to keep the parabola pointed directly at the sun, as well as protect your eyes from the intense glare.
I personally used an outdoor, propane barbecue to provide the heat to drive off the remaining water (Figure 4).
If you want to minimize the use of propane you can use a pot which is insulated around the sides (not the bottom of course) with heat resistant insulation in order to reduce convection heat losses to the ambient air from the outer surface. Also, using the outdoor barbecue on a warm summer day further reduces the level of convective losses to the ambient air, relative to say, a cold winter day.
The brine gets very hot, hotter than boiling water. So be careful. You will find that the salt crystals dissolve into liquid form as the brine gets hotter, and eventually will start to boil, reaching a temperature of up to 150 degrees Celsius.
As the remaining water is slowly evaporated off you will notice the salt start to crystallize, beginning with a skin on top, followed by larger clumps solidifying at the bottom. Once this starts to happen it is a good idea to stir the brine. You don’t want it to solidify into one solid layer, as it can be difficult to remove it from the pot afterwards. Another point to make is that, by not stirring you risk pressure buildup in cavities underneath the layer of salt. I heard a few loud “popping” sounds as the trapped water explosively released pressure. Good thing the pot was made of metal!
But nonetheless, it’s good to use an old metal pot anyway, one that you don’t mind beating up to remove the salt afterwards. I had to use a rubber mallet to pound the chunks out of the pot I used, then breaking them up further with a hammer. After a bit of work I reduced the salt to reasonably small chunks. Not as small as the original pieces, but good enough to reuse. Note that I didn’t quite evaporate all the water, but the vast majority was driven off, which is good enough. See Figure 5 and 6.
It took about 10 hours total, which was about $5 worth of propane. Not too bad considering that I only have to do this once a year or so. And the bulk of the evaporation is done with the solar collector set up, minimizing the need for propane.
So after the initial purchase of the calcium chloride salt ($25), I am basically looking at an annual “operating” cost of $5-10, which includes the cost of propane and a small fan as shown here.
But in fact, you don’t even need to use propane, or any other heating source, to go beyond what the sun will evaporate on its own. The brine is already concentrated enough at that point, so you can re-use it as-is, as a dehumidifier.
Extracting Water From Air
As a separate side project I attempted to extract the water from the Calcium Chloride brine using a variation of the solar collector shown above. Doing this effectively is very useful because in some parts of the world water is a scarce resource, and having the means to efficiently extract water from air at low cost is invaluable. I believe that one of the best ways to do this is to use a relatively non-toxic desiccant, such as Calcium Chloride, to absorb atmospheric water vapor with little energy input (using only a fan to promote absorption). And afterwards you can use a source of high heat, such as from the sun, to extract the water again. This is very feasible in places of the world that are constantly hot and humid.
The set up I used is similar in principle to a Solar Still – for explanation see http://en.wikipedia.org/wiki/Solar_still.
I placed the same tray I used before inside a larger (stainless steel) tray, with sheets of styrofoam underneath the larger tray, for insulation, to reduce heat loss. The Calcium Chloride brine is placed inside the smaller tray. In the area between the smaller and larger tray I placed aluminum foil to prevent that area from getting too hot (since aluminum is a poor absorber of solar radiation). This is the area where the condensate will “fall” so it makes sense to keep this area as cool as possible. See Figures 7-9.
I then stacked bricks in the center of the smaller tray and placed a sheet of polyethylene plastic (the kind used as vapor barriers for buildings) on top (Figure 10-11), weighing it down on the edges with more bricks to form a pyramid shape, allowing water condensing on the polyethylene to slide down and fall directly into the area covered with aluminum foil.
Polyethylene is mostly transparent to thermal radiation making it a good surface to collect condensate (i.e. water vapor will easily form on the surface because it doesn’t get hot under the sun). Glass, on the other hand, tends to get hot under the sun making it more difficult for water vapor to condense on its surface. Figure 12 shows an earlier attempt in which I used panes of glass and sheets of polyethylene plastic. The water condensed on the polyethylene plastic but not on the glass.
I also used sheets of cardboard covered with aluminum foil, facing north/south, in order to reflect additional sunlight into the collector. I would re-align them every hour or so to ensure this was the case. See Figure 13.
I did get water condensing but unfortunately not a lot (Figure 14).
The main problem was that the brine reached at most 80 degrees Celsius. So even though the water was evaporating and condensing, it was not at a fast enough rate. To increase the rate you need more heat, pure and simple. And a way to do this, with solar power, is to use a large parabolic solar concentrator. This will increase the magnitude of heat flux into the brine a multitude of times. You can put a pot of brine (plus distillation type chamber), at the focal point of the solar concentrator, and as long as the parabola is pointed directly at the sun (perhaps using a tracking mechanism), you will get a high rate of evaporation and condensation. This can be scaled up to get a large amount of water per day, along with a scaled up version of what is shown here, to create the brine to begin with.
The alternative, of course is to use a fuel, like propane or (even better) a carbon-neutral fuel derived from biomass (like ethanol), as a source of heat, most recommended when the sun isn’t shining.