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Author: Subject: How effective is evaporation drying at room temp?
testimento
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[*] posted on 24-7-2013 at 10:24
How effective is evaporation drying at room temp?


I was wondering if I could make a shallow pan of couple of square meters and use it to evaporate larger amounts of liquids. How effective is evaporation at general room temps(18-25C), and if we have let's say 10mm layer of liquid at 1m2 surface(10 liters), how many days would it take to evaporate most of the water? Would using a fan speed up the process? I could also have an access to room that has piping underneath it with temps of 30-40C at general and I bet this would evaporate the water at a lot faster.

This is just to calculate energy savings to make liquids reasonably concentrated before putting them into oven for complete drying. Frying stuff with gas burner not only consumes expensive gas, but it also creates a collateral damage when boiling off water from dissolved salts that decompose at low temps.

Oh, just found something:


Quote:

In the US, the National Weather Service measures the actual rate of evaporation from a standardized "pan" open water surface outdoors, at various locations nationwide. Others do likewise around the world. The US data is collected and compiled into an annual evaporation map.[2] The measurements range from under 30 to over 120 inches (3,000 mm) per year.


This would indicate that the nominal evaporation rate is 2-8mm per day. According to this I could expect rates of maybe 1-2mm a day, so evaporating a 10mm layer would take 5-10 days at minimum. I think im gonna do some actual testing very soon.

[Edited on 24-7-2013 by testimento]
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MrHomeScientist
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[*] posted on 24-7-2013 at 10:40


Sounds like an interesting experiment to try.

Remember that evaporation rate also depends on humidity - if there's a lot of water already in the air, your solution won't have much incentive to evaporate. For experimental rigor, I'd place a barometer, thermometer, and humidity gauge nearby and record those values at regular intervals. One of those all-in-one weather stations would be a good choice for this.

Evaporation rate of liquids increases with increasing temperature, decreasing humidity, and decreasing air pressure. Pressure is probably a minor effect, compared to the other two. This would make a good science fair project!


Edit: Forgot to mention that yes a fan blowing a gentle stream of air over the pan would increase evaporation rate. I've never tried this myself, but from what I hear it's a considerable difference. My thought as to why is that you're blowing away the saturated air above the pan (as well as lowering the pressure somewhat). Also remember to cover your pan with something to prevent dust getting in. Cheesecloth is great for this.

[Edited on 7-24-2013 by MrHomeScientist]
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testimento
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[*] posted on 24-7-2013 at 14:34


The pressure would significantly affect the evaporation rate when passing into minor scale of millibar range, since the pressure acts exponentially: the bp of water at 500mbar is still 80C. Fan would create only mircobar differences, but the saturation of air is what causes it to function. A question is, how many watts would the fan need to have in order to be economically ideal - a small, 10W fan blowing for a week consumes a couple of kilowatts of energy.

I remember us growing cupric chloride crystals at school. We made a solution into petri dish and let it sit on the window for a week or two, and we could grow crystals of several centimeters in length. So sad I wasn't into the "boring and clinical" chemistry at that age, when energetic materials were of my interest. Those days I would sell my mother to get a tiny piece of sodium metal just to throw it into a lake, but nowadays when I've got a few kilograms of it sitting on my shelf under oil, I'd use it only for drying solvents and be horrified if someone even suggested throwing the chunk into a lake..

I came up with another kind of idea to radically improve the pan design: I make a plate, roll a few millimeters thick layer of cement or clay on it, wind a nichrome helix and place another layer of clay on top of it and put the pan on this surface. I can drive the power with few tens of watts and maybe get the water off within a day or two, or drive it at full power of 2kW, which would enable me to actually use the pan to boil off the liquid. The only function of the clay layer here is to act as an insulator for the nichrome wire.
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bfesser
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[*] posted on 24-7-2013 at 16:06


Again with the exaggeration! "A few kilograms of [sodium] sitting on my shelf," eh? Prove it. Seriously, your bragging is getting really annoying.



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watson.fawkes
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[*] posted on 24-7-2013 at 16:16


Quote: Originally posted by testimento  
a small, 10W fan blowing for a week consumes a couple of kilowatts of energy
A small 10 W fan consumes 10 W of power, no matter how long it's on. That fan blowing for a week consumes about 1.7 kW-hr in energy, which is a few cents in electricity for most people.

It's no wonder I've taken to avoiding you for the most part.
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[*] posted on 25-7-2013 at 23:29


Well I've had a wide (diameter about 35cm) glass baking dish sitting in my garage with a saturated solution of CUSO4 in it for around 2-3 months now. It has dropped in height by about 10-15mm in that time, which is much slower than the literature you posted, although I imagine the presence of CUSO4 could effect evaporation rates. The average temperature over that time period would be approximately 10 degrees.



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[*] posted on 26-7-2013 at 00:23


I use evaporation at room temperature quite a lot for recrystallizations. It takes some patience but it works nicely. Energy use is zero, you get nice crystalline material and it is simple. I even evaporate liter quantities in a shallow dish.

I do not use fans because that would introduce dust on the surface of the liquid. What I do is cover the dish with a piece of paper to assure no dust comes in from above. One way to do this is put two rods (with a diameter of a few cm) over the dish and then put a piece of hard paper (cardboard) over the dish. The size of the paper is larger than the size of the dish, it covers the dish completely with rims extending at least several cm over the rim of the dish. The rods assure that there is a few cm of space between the paper and the rim of the dish such that vapor can escape. The paper only slows down the evaporation process a little bit, but it assures that only very little dust settles in the liquid.

If I want things to speed up a little, then I put the dish on our central heating furnace. At the top, this device heats up a little (maybe 35 C or 40 C when one of us takes a shower or when the heating systems fires up for a while) and this helps speeding up the evaporation process and it also is useful to dry a still somewhat humid crystal mass.
For this purpose I have a dish with extra high rim, to reduce the risk of pouring some water on the furnace by accident.

[Edited on 26-7-13 by woelen]




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testimento
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[*] posted on 30-7-2013 at 14:20


I conducted a test with evaporation with my fertilizers recently. I put the partially crystallized liquor in a shallow pan and used black/white colored plastic sheet, black up, white down, to condense the heat from sun. So far the liquid layer has noticeably decreased, and it should take few days to evaporate it to mushness.
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[*] posted on 3-8-2013 at 09:18
Yes. It is super effective.


I made an evaporating pool from plastic sheet on an even surface lined with planks with surface area of about 2 square meters and poured about 20 liters, or 10mm depth of liquor to it. During 6 hours at direct sunlight on 25C ambient temperature and slight wind, a substantial evaporation rate of about 6-7 liters can be detected. Some of the contents have begun to crystallize on the pool, as can be noted on the picture.

pool1.jpg - 199kB

I'm currently conducting another test with similar specs inside a building with floor heating to see whenever the evaporation drying is appliciable in circumstances drying outside is not possible when the operating conditions are limited by resources or political situation. I have a room of 2x3 meters, or 6m2, triple the amount available for testing and I will report the consequences.

[Edited on 3-8-2013 by testimento]
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[*] posted on 3-8-2013 at 18:07


Regardless of anything else you do, I can definitely say that yes a fan will speed up the process.
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[*] posted on 3-8-2013 at 20:47


Good work...

surface area is one of the variables in evaporation. the fan also helps to increase mass transfer. Black is also a good color choice.

I like the surface area as a variable to exploit vs. using electricity or fuel to heat the fluid.

If your interested in keeping the vapor it adds complexity of course, but in the case of water I guess you can let it go and get your scrapings!
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[*] posted on 4-8-2013 at 06:13


I got so sick of evaporating at STP I built a rotary evaporator. Dust would fall into the trays, aqueas solutions would get moldy, chemicals would oxidize, etc.

Another trick I learned when I used to evaporate in open air was to cover the trays with a T-shirt or paper towels snugly. This helped prevent dust and small particulates from entering solutions. Increased evaporation times however.




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testimento
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[*] posted on 4-8-2013 at 12:46


Ok, the job is finished now. The rest of 20 liters of water evaporated off today and I collected the still-moist crystals in a 10-liter bucket, obtaining about 9 liters of product. It will be final dried in an oven to obtain anhydrous product.

I also had another eva vessel which I covered with the same type of plastic. The sheet is black and white, placed black up white down, so the heat is absorbed and irradiated back to the black container. This vessel had about 5 liters of partially crystallized product with surface area of about 0.25 square meters and it took about 5 days to obtain same results with 4 times less liquid than the sheet pan. This is not directly comparable because the first 2 days were quite cloudy and cold and even some precipitation occured, although the plastic sheet cover prevented it from accumulating to the vessel.

The obvious disadvantage of this method can be seen in the pictures noting some (nitrate-eating) insects and airborne impurities that will contaminate the product. It could be partially prevented with very thin fiber used in protecting vegetables in agricultural industry. This fiber will let moisture through like nothing, but it stops even very fine particles. I still consider open pan evaporation a crude method that will result in a technical quality product, although still suitable for most amateur purposes.

The product consists mainly UAN at 40:60 ratio, so the next project is to separate the urea and AN. I've looked for several methods and very few seem to be utilizable, so I probably just mix the whole goo with a good bunch of CaOH and collect the ammonia gas, leaving in theory only calcium nitrate left, which could be then used for several purposes.

poola2.jpg - 101kB

evapot1.jpg - 86kB

evapot2.jpg - 79kB


EDIT

Another test conducted inside a building, this time conditions are similar in temp, but a fan was being used on the later phase. This evaporation consists of 1.5 liters of liquor with about 200 grams of dissolved goods, and the pool is about 0.35m2, or 4.2mm depth of liquid. The evaporation rate without the fan was about 30% of total within 2 days, but after adding the fan, the evaporation rate has increased dramatically, to as much as 50% of original volume decrease within just 5 hours.

Conclusion: surface area is the major factor of evaporation rate, second most important seems to be the air flow, increasing the rate substantially up to certain point, and finally the temperature. The temp effect will be major factor, of course, when the temp is brought high enough, but at ambient temperatures the first two seem to be the most prominent factor. Evaporation seems to be very effective method for removing water from process liquids even for amateur chemist operating in small scale. When living in a hot country close to equator, the effects of evaporation are yet dramatically increased.

eva1.jpg - 63kB


[Edited on 5-8-2013 by testimento]
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