Creating aerogel with supercritical methanol

In previous videos, I used supercritical CO2 to dry my homemade aerogels. This time, I soaked the aerogel in methanol, then raised the temperature and pressure of the methanol itself to make it supercritical. This allows the gels to be dried without an additional solvent exchange into CO2. The downside is that it requires a chamber full of methanol at 460*F and over 1200 psi, which is a much bigger hazard than using CO2.

The aerogels dried with methanol shrank less than the ones dried with CO2, but there was still a lot of cracking, and I have yet to create a high-quality monolith.

Caffeine extraction from green coffee with supercritical CO2

I finally succeeded in extracting caffeine from green coffee beans by using supercritical CO2. I built a high pressure chamber from 2" steel pipe fittings, and poured in 200mL of water. There is an aluminum screen above the water line, which held 0.75 lbs of moisturized green coffee beans in the upper part of the chamber. I added liquid CO2 to the chamber, then closed all valves and raised the temperature, making the CO2 pass into the supercritical phase. I left the system overnight at about 60*C, 3000 psi, then drained the water. It was very black due to impurities and some bean burning that occurred where my electric strip heater caused localized overheated zones in the chamber. The water was highly caffeinated, and tasted somewhat like coffee. I used a typical hydrocarbon extraction process to isolate the caffeine from the water (will show this in a later video).

Trying to visualize beta particles in supercritical CO2 (still no success)

In an earlier video, I tried to visualize alpha particles in supercritical CO2, similar to an isopropanol vapor cloud chamber. Someone commented that the alpha particles will not travel very far (maybe 10 microns) in liquid or supercritical CO2, and recommended that I try beta particles, which should have a path length of almost 10mm. Unfortunately, I still don't see any bubble or droplet trails using strontium-90 and thallium-204 sources. It's possible that the ionizing effect of the radiation particles does not interact with the CO2 phase change as it does by condensing droplets in a cloud chamber. Also, cloud chambers are very finicky, and if this CO2 visualization method is as finicky or worse, it may take some more time to figure out the right combination of environmental variables.

Making silica aerogel at home

I followed instructions in the silica TMOS recipe from http://www.aerogel.org and successfully produced some small pieces of aerogel in my home shop.

The two main difficulties are: 1. Getting TMOS or TEOS (the key chemical ingredient), and 2. Building a supercritical drying chamber. The components for the chamber can be bought from http://www.mcmaster.com or another source of industrial pipe fittings. You'll also need a supply of liquid carbon dioxide. I used a 20-lbs cylinder, which I bought from a local welding store. Most of the cost is in the cylinder itself, since a refill costs only $20 to $30. You may find a welding supply shop that will rent the cylinder.

Getting the TMOS is difficult since chemical suppliers are generally unwilling to sell to individuals.

The process to make aerogel is:

1. Mix TMOS, methanol, and ammonium hydroxide. Pour this mixture into molds, and wait for a gel to form.
2. Submerge the gel in methanol, and wait a day for the remaining water in the gel to diffuse into the methanol.
3. Discard the methanol, and replace with fresh methanol. Wait a day, and repeat. Repeat this process a few times over three days.
4. Transfer the gel into the supercritical drying chamber, and fill the chamber with methanol.
5. Add liquid CO2, then open the chamber's bottom valve to remove the methanol. Make sure the gels are always covered with liquid CO2.
6. Wait a day for methanol to diffuse into the liquid CO2.
7. Open the bottom valve and remove more methanol.
8. Repeat the methanol draining procedure while making sure the gels stay submerged in liquid CO2. Repeat the CO2 draining/exchange a couple times over 2-3 days.
9. Raise the chamber temperature to cause the CO2 to become supercritical. Slowly vent the chamber while applying heat to ensure the CO2 moves from the supercritical phase to the gas phase. Continue venting the chamber slowly, then remove the finished aerogels.

Supercritical CO2 does not help visualize ionizing radiation

I tried to build a cloud chamber with supercritical CO2, thinking that ionizing radiation (alpha particles) would cause localized condensation of the CO2 at the point where the fluid is coming out of the supercritical state. It didn't work, unfortunately. I tested this idea with the americium-241 source from a smoke detector. I will continue experimenting with CO2 ionization chambers, and it might be possible to visualize the particles with superheated liquid CO2.

A helpful commenter pointed out that alpha particles will not travel very far in a fluid as dense as liquid CO2, so I will try again with a beta emitter.

Supercritical drying chamber for aerogel production

I built a pressure chamber from 2" pipe fittings and 1/8" brass valves to contain supercritical CO2 for drying applications. One project is to try aerogel production which generally requires that solvent be removed via supercritical drying. Normal evaporation would deform the aerogel structure as the surface tension of the solvent pulls the gel's structure tighter together and makes it dense. Since supercritical fluids have gaseous properties, they can diffuse out through the gel without affecting the structure the way that a liquid would.

Supercritical CO2 caffeine extraction (negative result -- more work needed)

I tried to extract caffeine from green coffee beans using supercritical CO2, but I had no success. The beans underwent a strange transformation, becoming white and rubbery after 6 hours at 80*C in supercritical CO2. I also used water and ethanol as a cosolvent, thinking that the caffeine would end up in solution in the water/ethanol mix after the CO2 became subcritical.

Do you have any advice about how this process is supposed to work?

A close look at supercritical carbon dioxide CO2

I built a pressure vessel from aluminum and acrylic and filled it by placing pieces of dry ice inside. The dry ice melts under high pressure, and forms a liquid and gas phase. When the vessel is heated, the CO2 becomes supercritical -- meaning the liquid and gas phases merge together into a new phase that has properties of a gas, but the density of a liquid.

Supercritical CO2 is a good solvent, and is used for decaffeinating coffee, dry cleaning clothes, and other situations where avoiding a hydrocarbon solvent is desirable for environmental or health reasons.

If you have a suggestion for what I should do with the supercritical CO2, please leave a comment.

Here are a few engineering calculations that I used to determine the pressure capacity of the chamber:

1. Hoop stress in the aluminum ring:
http://www.engineeringtoolbox.com/stress-thick-walled-tube-d_949.html
The aluminum alloy and heat treatment is unknown unfortunately, which makes a huge difference in its material properties. Since it is a structural tube, I will assume 6061-T4, which has a yield strength of about 40 ksi.

Inner radius = 1.1", Outer radius= 1.5" (to the inner edge of the bolt circle)
Chamber pressure = 3000 psi
Hoop stress at inner edge = 10ksi

So, there is a safety factor of 4, but the additional material outside the bolt circle will actually add to this factor. In theory, the aluminum will yield at 12000 psi chamber pressure.

2. Bending force on the acrylic windows:
Acrylic ultimate strength: 10 ksi. It doesn't yield. It is elastic, then breaks. Modulus: 400 ksi
http://www.efunda.com/formulae/solid_mechanics/plates/calculators/cpS_PUniform.cfm#Results
The plate is not a thin plate, but the results show only a 0.004" deflection at the center under a chamber pressure of 3000 psi.

http://www.xcalcs.com/cgi-bin/tutti/x3calcs.cgi?d=i_4_0_1_0_0&l=en

This shows a stress of about 4.3 ksi for a 1.25" thick acrylic plate with 1.35" radius. The pressure-bearing radius is larger than the inner radius of the aluminum ring. This has a safety factor of 10/4.3 = 2.3. In theory the acrylic will break apart when the chamber reaches 7000 psi.


3. Stress on the bolts:

Total window area is about (pi)(1.35)^2 = 5.7", so total force when chamber pressure is 3000 psi is (5.7)(3000) = 17,200 pounds! I will use six bolts, so each bolt must hold 17,200/6 = 2860 pounds.
http://www.derose.net/steve/resources/engtables/bolts.html

1/4-20 bolts are NOT strong enough -- even at grade 8!

5/16 bolts would be OK in grade 8, but I wanted a higher safety margin, and I don't like 5/16 bolts.

I chose 3/8" grade 8 bolts, which have a working load of almost 7000 pounds. I wanted to be sure bolt failure could not possibly be the failure mode that breaks the whole system. I also used grade 8 nuts, which should ensure the failure happens within the fastener, not by shearing the threads out of the nut or bolt. I am not positive about this, though.

4. Pipe threads:

I wasn't sure what 1/8" pipe threads are capable of holding, but McMaster sells such fittings that are rated for 5000 psi (like the gauge that I used), so I assume a brass part can hold such a load. I cut threads into the aluminum so it's possible that the pipe thread in aluminum could fail (ie the gauge or valve could be pushed out, shearing the threads right out of the aluminum ring). It might be possible to add up all of the area of the pipe thread cross-sectional area, but it seems silly and unlikely to be at all accurate.

5. Temperature concerns:

The acrylic has a glass transition temperature of at least 180*F, but it should not be heated anywhere near this temperature or else its ultimate strength rating may not be valid. I would say 130*F is the upper safe limit.

6. Effect of supercritical CO2 on the acrylic and O-ring:

I used buna-n O-rings, which may affected by exposure to SC CO2. They are very unlikely to fail in the short term, and I can change the O-rings for every experiment if I want.

The acrylic showed signs of crazing after just one supercritical CO2 cycle. I think the crazing is unlikely to affect the acrylic's ability to hold pressure, but there is a slight concern.


The most likely failure mode would occur when the acrylic reaches its ultimate strength, and suddenly breaks. Unlike pressure vessels made from ductile materials, which can be designed to yield and leak before breaking, the acrylic will suddenly blast apart without leaking first. If the equations and material specs are correct, 3000 psi should be OK, but I would not want to go much higher.