vacuum drying chamber

[MadMat]

Has anyone ever tried making/using a vacuum chamber for drying out chem.'s or stars ect. ?

I have an old compressor from a window air conditioner that makes a great vacuum pump. I have used it in the past for pulling a vacuum doing some fun stuff with electronics (think about those glass balls with the colored electrical streamers in them. They are commonly called plasma globes) A small chamber could be easily made from a thick walled glass fish bowl. Just lay some wet/dry silicon carbide paper on a flat surface and rub the fishbowl top on it. Start at 120 grit and go up to 320. This will make a nice flat machined surface on the top of the fish bowl that will seal up on some 1/4" plexiglass with a little beeswax rubbed on it. You can drill and tap a hole in the plexiglass for a fitting and connect it to the intake side of the compressor. Just make sure you put a "T" in the line with a gas ball valve. Compressors are filled with oil and if you shut off the compressor without releasing the vacuum, whatever you have inside the chamber could get an oil shower. If anyone has any experience with vacuum drying, please let me know if its worth the effort before I begin making mine.

Edited August 3, 2015 by MadMat

[schroedinger]

It certainly works, but the problem is that the szars cool during evaporation and vacuum is a good insulator, only a small vacuum is working.

[Smokey]

It certainly works, but the problem is that the szars cool during evaporation and vacuum is a good insulator, only a small vacuum is working.

Would you expand on your thoughts as to why this isn't an efficient method? Thanks.

[Col]

The water needs energy to evaporate aka change phase from a liquid to a gas, it takes approx 1000btu per lb. The required energy comes from the air in the fishbowl, which in turn causes a drop in temperature and a rise in relative humidity. Cold, saturated air isnt great for drying stuff. A little heat and plenty of airflow would be better.

[MrB]

The vacuum causes evaporation. Evaporation causes cooling. Cooling will cause the stars to freeze, if there is enough water in them to start with. I somehow doubt that is the case. But if there is, freezing would cause (micro) fractures. The ice would still sublimate and be given of as steam, so you could essential freeze dry the stars from that point on.

The cold might make the binder not function as intended, the fractures might make the stars brittle. But i have no idea at this point, it's theory. In reality i don't see why it wouldn't work. I'm sure there is a number of reasons why it might not work, but i just don't know em. Try it if you have the gear?

 

The "driven in" problem some people seam to have where stars dry to fast on the surface, just to then never (well, that's reportedly what it feels like) dry completely, could be an issue as well. I've never had that issue, but for some it seams to be a near constant battle.

B!

[Mumbles]

Cooling is the biggest issue as others have mentioned. Once the stars cool off, they're not as efficiently dried anymore. Some people have played with vacuum ovens and clear vacuum containers to allow the use of lamps to warm things back up. Nothing really works all that well unfortunately. You also apparently need to be able to shift the stars gently so that all sides are heated/exposed to light for this to be even moderately effective.

 

It's hard to beat air flow and gentle warming. If you were looking to speed up your processes, I'd look into a drying box.

[CaverCork]

I see where this is going and the issue. The lower the air pressure, there is less force exerted on the molecules thereby causing faster evaporation, think tropical storm or boiling water for high altitude cooking for examples. Less heat energy is required for evaporation to occur so it appears on the surface to be a good idea. The problem is the closed system and saturation. Once the remaining air in your vessel (provided you don't pull a full vacuum which would accomplish almost nothing) is saturated with whatever you are trying to evaporate, the process will slow or stop. You still need to replenish the system with unsaturated air to continue the evaporation process. A drying box will be far more efficient.

[Fulmen]

Before setting up a test I would do a thorough search on the net, it's probably tried before.

 

I see several problems with this approach, but it might work if you get things right. First of all there is the problem of boiling if the pressure drops too much. This will probably cause the stars to swell and break apart. If you freeze them before reducing the pressure you will prevent this, but freezing will most likely cause swelling in itself and I doubt if the binders will work in such conditions.

 

But luckily you can drop the pressure quite a bit before boiling occurs, at room temperature water boils at 20-30 millibar while alcohol boils at around 50mbar. At these low pressures air convection will not transfer much energy, but radiant heat will still work. This could be a light bulb inside the enclosure or even outside if the enclosure is transparent.

 

I can't tell you if it's going to work better than forced air, but it's worth exploring.

[Col]

Looking at it from a saturation viewpoint, if you have a vessel with a volume of 1 cubic foot (7.5gal) containing air at 68F and 50% relative humidity. it would take less than 0.25g of water to raise the relative humidity to 100%. That assumes the sensible heat required to evaporate the water isnt provided by the air in the vessel, which would cause it to cool and hold less water. With a 1 gallon vessel, you`d have to cycle the air 30 times to remove 1g of water from the stars

[MrB]

The only way i see vacuum being effective is with a near complete vacuum. A middle ground is useless as it will just get filled with the water-vapors, and then it's just not changing anymore. The pressure-drop causing boiling i hadn't even considered, but that to might be an issue. If it doesn't sublimate it will boil, until it freezes, and starts boiling. Again i suppose testing is the only way to know what happens there. If it does boil, that will be a killing blow, but quite frankly, i think the micro-fractures from ice-crystals, and the binder not being able to function in the cold will be enough to keep this from being a great idea.

If one wants to go high tech, i think dry-pressing is the way forward.

B!

[Fulmen]

You're missing a crucial point here. The evaporated solvent will increase the amount of gas in the chamber, if evacuated and sealed the pressure would quickly increase towards ambient. But if the pump is left on the solvent is simply pumped out, allowing continuous evaporation.

[CaverCork]

Water can only exist as a solid or gas in total vacuum, depending on temperature. If liquid water at any normal Earth temperature is suddenly put in a total vacuum it will boil. This would most likely be undesirable in the drying of stars. It may not be bad for drying powder chemicals though if it doesn't change the sublimation temperature point of the chemical in question.

Edited January 19, 2016 by CaverCork

[Fulmen]

An absolute vacuum (or anywhere near for that matter) is pretty hard to produce and would indeed cause problems. But 50-100mbar is within reach with a fridge pump and would still allow water to remain liquid.

[Col]

A decent 3/4 hp light commercial compressor will do around 25" of mercury. I use one on a homebrew silent air compressor

[MrB]

If liquid water at any normal Earth temperature is suddenly put in a total vacuum it will boil.

 

It will indeed boil, but only for as long as it takes to freeze. The boiling is gas-bubbles, the energy needed to go from fluid to gas is a constant, and that energy is taken from the remaining water, lowering its temperature.

[Fulmen]

25" of mercury should equate into appr. 150mbar, well within the range where water remains liquid at room temperature. The cooling effect could still freeze water if the pump has enough capacity, but that is easily solved by either regulating the flow or bleeding to atmosphere.

[Mumbles]

If you have a good pump, and pump on water it will boil for a little bit then freeze and just start sublimating. The sublimation usually soaks up enough energy that it will often maintain the solid state without some sort of external heating. This is dependent upon the vacuum obtainable though. Subliming, or melting and vaporizing take the exact same amount of energy, so it's just a matter of rates. I do agree that causing the water to either freeze or boil is probably not good for the star's integrity.

 

When you get down to it, we are trying to convert water into vapor and remove it from the star. There are a few ways to do this.

 

1. Decrease the pressure, which is what we're talking about doing here. As the surrounding pressure decreases, the partial pressure of the water increases. This increases evaporation rate. If you have a pump capable of evacuating 1 cubic foot of air per minute, if that same cubic foot contains 30% more water, it will evaporate faster.

 

2. Heat. As the temperature of water increases, so does it's vapor pressure.

 

3. Airflow. Vapor pressure is the equilibrium pressure exhibited by a liquid. At room temperature, water has a vapor pressure of .0231atm, which also corresponds to 100% relative humidity (RH). Locally around a drying star, it could be assumed that this is the vapor pressure/relative humidity. If the air you're blowing over the star comes in at 50% RH, then it will try to saturate the air local to it. If you used continued airflow, the water will continue to evaporate.

 

4. Mixtures of solvents, normally alcohol and water. Honestly, this doesn't really do a lot with the concentrations of alcohol that are useful. The alcohol (ethanol specifically) will evaporate faster than the water, though takes a little water with it. I could bore you with the back of the envelope math, but 10% alcohol will speed things up about 6%. 25% alcohol is around 17%. This doesn't take into consideration the enhanced cooling of the stars, which will lower the effective drying rates. 2.5 vs 3 days to dry something is basically inconsequential to me.

 

5. Dehumidification. The drier the incoming air, the more water it can soak up and the stronger the equilibrium driving force for evaporation is.

 

In the best of all worlds, you want some combination. Using a vacuum eliminates some of the option, and adds increased challenges. The best case scenario is probably running a dehumidified drying box on top of a mountain. Good luck.

 

By the way, most pumps are not really designed to take in solvents, so unless you're adding a cold trap (which also helps to drop the "humidity") or have a solvent suitable pump, this isn't really a viable long term option.

[CaverCork]

Boiling, freezing, or both, still a disaster for drying stars unless the draw down and temperature is carefully controlled. Not to say this method won't be of any use. I already have a few experiments that I want to try. Drying chemicals is the first and the other I won't mention because a semi related thread (relational only to my warped outside the shell thinking) I was commenting on/following disappeared. I think it went to HE land. Which is probably where it should have been in the first place.

Edit: Darn, I was writing this as Mumbles posted. You have a point about the pump. If it is oil based it's going to have a real bad day with most anything other than water which obviates my second experiment before it starts.

Edited January 20, 2016 by CaverCork

[schroedinger]

O ly thing that sounds viable using ghe vacuum method, is a closed chamber with two openings. Onto the first you attach the vacuum pump, the 2nd is becomming an air intake. Now instead of lowing hot air into the chamber, you pump air out. The intake gets restricted this much, that you maybe get a pressure decrease by .3 bar.

[lloyd]

Or you could just eliminate the intake, and put an adjustable vacuum switch on the pump, with some hysteresis between on and off points, so that it does not cycle the pump too rapidly.

 

Then it would pump down to the set point and shut off. When vapor pressure inside the vessel caused the pressure to rise to the on-point, it would again pump down, etc, etc.

 

When the pump stops cycling on, the batch is dry.

 

Lloyd

[schroedinger]

Lloyd, you could do that, but your setup has ond big disadvantage. Either you need to run it at a very low pressure, where you again get problems with the cooling and if you run it at moderate pressures it helps nothing, since the hummidity would raise and bring the liquid phase towards an equilibrium with the gas phase (every pump ci le would remove air and water molecules, but both will just be replaced with evaporating water).

 

But you could make the setup even more effective, by using a rotating drying screen, inside a recuced pressure chamber, with continous air flow. (And if you do so we are again bringin this discussion into direction of ad absurdum)

[lloyd]

"Either you need to run it at a very low pressure, where you again get problems with the cooling and if you run it at moderate pressures it helps nothing, since the hummidity would raise and bring the liquid phase towards an equilibrium with the gas phase (every pump ci le would remove air and water molecules, but both will just be replaced with evaporating water)."

---------------

Heh! You _completely_ missed my very point! You can cause the "equilibrium point" to be at the -0.3 bar you recommended with a relatively simple vacuum switch!

 

My exact point was that it's the very process of evaporation that you want to sense -- directly. And a vacuum switch with a little hysteresis to protect the pump motor performs precisely that function without any adjustable intakes. Besides, such an intake's setting would have to be continually changed during drying to account for the decrease in evaporation of the water, or else it would go to higher and higher vacuum as the drying proceeded, since the water would no longer be contributing to the 'makeup' volume of gasses.

The switch would keep the vacuum level constant (within its hysteresis range), allowing it to run completely unattended. When it stops, it's done.

 

Lloyd

[schroedinger]

Lloyd the makeup volume of water is nothing you really need to consider, it is a tiny volume compared to the volume of air sucked through. 18 of water make up for 22 litres. Even with TT stars you would get a lousy 220 l / kg comp of volume that the water contributes. Evaporated over 24 hours ( if you can get tt dry in 1 day). A small pump would pump ahout 3 l/ min at these pressures. Meaning 4.3200 l / 24h. Considering that you use 5 kg the water would only contribute 25% of Volume.( And a 3/l min @ .7 bar pump would also pump more volume if the pressure raises.)

 

Combine that with the fact that with a normal fish tank pump allready pumps more air and an oil pump does about 40 l /min.

[lloyd]

Nobody's listening.

 

I'll shut up now, and try to figure out why the two vacuum pots (and three pumps) I've used for over twenty years don't work like I thought they did. I learn something new every day.

 

LLoyd

Edited January 21, 2016 by lloyd

[Smokey]

Having vacuum dried many things and even freee-dried a few, but never stars, in the lab, I'm missing something in this discussion.

 

Vacuum chambers I'm familiar with are not normally closed systems. The chamber is continuously being evacuated by the pump: their is no inlet if you're just trying to remove as much moisture as possible. Heated vacuum ovens are also avaliable.

 

"Boiling" is a concept best reserved for bulk water. For the normally small amounts found in star comps obviously there will be a phase change from liquid to gas but I'm skeptical there will any associated turbulence as would be found in boiling water.

 

"Drying in," a term usually associated when film forming binders are of sufficient concentrations as to form a more/less impervious shell, blocking diffusion of water from the interior of the star, should be avoidable.

 

If you have vacuum oven system I'd experiment with it. Keep track of type/amount of binder and the amount of water and share the results.