Supercritical Drying with Liquid Carbon Dioxide Part 1 of 2

By this point, we assume that you

• Have read about aerogels and aerogel production
• Understand what supercritical drying is
• Understand why you need to do it, and
• Understand why liquid carbon dioxide is the preferred solvent and that elevated pressures are required to use it

We also assume you have:

• Built a supercritical dryer like the one presented in the Make section, bought a commercial supercritical dryer from somewhere, or have access to a supercritical dryer in a lab
• Prepared gels of some sort to make into aerogels, and
• Exchanged the pore fluid of your gels (multiple times) into methanol, ethanol, isopropanol, acetone, hexane, or some other liquid that is miscible with liquid CO2

This page will focus on how to use the design presented under the Make section here on Aerogel.org but if you’re using a commercial supercritical dryer or a different design you can probably figure out the gist of what you need to do for your system.

Getting Started

Now would be a good time to read over the following articles if you haven’t already and make sure you’ve done everything in them:

• Selecting a Workspace
• Ensuring Safe Operation
• Setting Up the CO2 Tank

All that said and done, let’s make some aerogels!

Stuff You’ll Need

• Supercritical dryer
• Tank of liquid CO2 with a siphon tube (for example Airgas part number CDS 50)
  
• A trusty pipe wrench and/or crescent wrench with a 2″ grip
• A freezer large enough to fit the supercritical dryer inside (see note below)
• PTFE (Teflon®) pipe thread tape
• Gels ready for supercritical drying, soaking in a solvent bath
• Solvent-compatible spoon, scupula, or spatula (to pick gels up with)
• Disposable syringe
• A crumb cup (with holes in the bottom and sides) or wire cage to put gels in
• Tweezers or pliers
• Lint-free paper or cloth towels
• Paper towels for wiping up spilled solvent
• Solvent-resistant glass or plastic cup

If you don’t have a window in your supercritical dryer, you may also want:

• A moderately-precise scale with about 25-50% more capacity than the weight of your supercritical dryer
• A volumetric measuring device such as a 100-mL graduated cylinder

And if you have a supercritical dryer with a window:

• An incandescent flashlight

A Few Notes

It’s Basically Just a Hyped-Up Bottle!

Your supercritical dryer is essentially a high-pressure bottle in which you can store and manipulate high-pressure fluids such as a liquid carbon dioxide and supercritical carbon dioxide. Keep that in mind and it will be a lot less intimidating.

T-3 Days to Aerogels?

The entire process for making aerogels involving your supercritical dryer takes about 3 days. (3 days?!)

Most of the time your gel is in the supercritical dryer, it is not being supercritically dried. Rather, it is soaking in liquid CO2 to exchange the pore fluid inside the gel with liquid CO2. Essentially, you are performing another solvent exchange, except instead of using ethanol or acetone in a jar, you’re using liquid CO2 in a high-pressure vessel. Like all solvent exchanges, this solvent exchange is diffusion-controlled and takes on the order of 3 days (with at least one exchange per day) for the sizes of gels that can fit inside the Aerogel.org designs.

Supercritical drying, then, actually only takes about 1-4 hours.

In short, the entire process can be broken down into:

• Phase 0: Loading (1 hour)
  
• Phase 1: Solvent Exchange into Liquid Carbon Dioxide (2-3 days)
  
• Phase 2: Supercritical Drying (1-4 hours)
  
• Phase 3: Depressurization and Shutdown (1-2 hours)

What Do I Need a Freezer For?

Making your life a lot easier, that’s what. In performing solvent exchanges with liquid CO2, you will need to drain the liquid CO2 out of your vessel (without depressurizing it) and refill it with more liquid CO2 by siphon action several times. The following section will help you to understand this process better.

Understanding Siphon Action (You Need to Know This)

Siphon action is the process by which liquid carbon dioxide is carried from the gas tank into your supercritical dryer.

Understanding siphon action is really important. It will help you to build an intuition of what to do when. And to understand siphon action, you need to understand vapor pressure.

Vapor Pressure

Every liquid has what’s called a vapor pressure–that is, liquids create a gas above themselves and that gas has a certain pressure. For example, if you put water in a soda bottle at room temperature and seal it up, some of the water will evaporate to make water vapor in the bottle until the vapor pressure of the water at room temperature (0.4 psi, 20 mmHg) is reached. At this pressure, the amount of vapor recondensing to a liquid and the amount of liquid evaporating into vapor is the same (the liquid and vapor phases are in equilibrium).

The vapor pressure of a substance is solely a function of temperature and in general increases with increasing temperature. So if you put a blob of water in a soda bottle and seal it up, the total amount of visible liquid water will decrease with increasing temperature since the amount of water in the vapor phase increases.

The Vapor Pressure of Liquid CO2

The vapor pressure of liquid CO2 ranges between 600 psi in a cold garage to 850 psi in a warm laboratory. This is substantially larger than the vapor pressure of water in a soda bottle (0.4 psi). For reference, atmospheric pressure is 14.7 psi. As a result, we have to contain liquid CO2 inside of strong metal containers (gas tanks and supercritical dryers and the like).

Vapor Pressure Can Be Different in Two Different Tanks

If two tanks containing liquid CO2 are at the same temperature (for example, both tanks have been sitting in the same room for a while), the vapor pressure in both tanks will be the same.

But if we chill one of the tanks, the vapor pressure of CO2 in that tank will be lower than the other.

Imagine one tank is the gas tank you got from Airgas (or whoever). The other tank is your supercritical dryer. The gas tank contains liquid CO2 at 20 deg C and as a result the pressure of CO2 above it is about 750 psi. The supercritical dryer also contains some liquid CO2 but is at 15 deg C. As a result the pressure of the CO2 gas above the liquid in the supercritical dryer will read about 700 psi.

Why Your CO2 Tank Has to Have a Siphon Tube

The liquid in your CO2 tank sits in the bottom half of the gas tank. Above it is CO2 vapor. If there is no siphon tube and you open the valve of the gas tank, gas will come out because that’s what’s going up into the valve.

If there is a siphon tube, however, when you open the valve of the gas tank, the CO2 gas (at 600-850 psi) will push down on the liquid and force it up through the siphon tube (like soap coming out of a soap dispenser). Now it’s liquid that is fed into the valve of the tank, and that’s what will come out.

Of course, if the pressure of the realm the liquid is entering is less than the liquid’s vapor pressure–for example, say you’re just opening the gas tank to your garage, which is at 15 psi–the liquid coming out of the tank will instantly boil in an attempt to create a world with a pressure high enough for it to exist as a liquid. As a result you will just get gas, and some dry ice (which is what happens when CO2 vapor is cold and concentrated at atmospheric pressure).

This is a selfless act–the liquid CO2 boils itself so that future generations (the liquid CO2 coming after it) may have the opportunity to enjoy life as a liquid outside of the gas tank. Alas, in the relatively infinite volume that is your garage, this act is in vain, as a pressure greater than 15 psi will never be attained by such a small gas tank.

But if you have a supercritical dryer attached to the gas tank instead, the liquid can now enter the dryer, boil, and successfully increase the pressure of this new realm, eventually reaching approximately the same vapor pressure in the gas tank. In fact, the CO2 has created a world inside the supercritical dryer safe for liquid CO2 to exist.

Siphon Action Happens When There is a Pressure Differential

When you open the gas tank to atmosphere, the vapor pressure inside the CO2 tank acts like an invisible piston pushing down on the liquid. This is what causes the liquid to be forced out of the tank. BUT if there is another invisible piston pushing back on the liquid, no liquid will come out of the tank.

Say again we have our gas tank attached to our supercritical dryer and we open the valve on the gas tank. Liquid will move from the gas tank towards the supercritical dryer and instantly boil until the same vapor pressure in the gas tank is reached.

Now there is an invisible piston in the supercritical dryer applying the same amount of force in the opposite direction of the invisible piston in the gas tank (both tanks are at 750 psi). Generally speaking, no liquid will flow (the tanks are at equilibrium).

Oops. That sucks. We need liquid in the supercritical dryer.

Let’s now cool the supercritical dryer to, say, 15 deg C. Here’s what happens (pay attention):

• The gas tank is at 20 deg C, and so the vapor pressure of CO2 in the gas tank is 750 psi
• The supercritical dryer is at 15 deg C, and so the vapor pressure of CO2 in the supercritical dryer will be about 700 psi
• CO2 entering the supercritical dryer will now boil until a pressure of 700 psi is reached, since that’s its vapor pressure at 15 deg C–any additional CO2 pushed into the supercritical dryer can now exist as a liquid
• The vapor pressure in the warmer gas tank is 50 psi higher than the supercritical dryer
• Now a pressure differential of 50 psi pushes down on the liquid in the gas tank, pushing liquid through the siphon tube into the supercritical dryer, where it no longer needs to boil since it is entering a realm already as saturated with CO2 vapor as it needs to be
• Liquid siphons over until the supercritical dryer is full

Phew. And that’s how siphon action works.

Ways of Creating Pressure Differentials

There are a couple of different ways to create pressure differentials between the gas tank and the supercritical dryer to get liquid CO2 to siphon over. Here are some:

• Cool the supercritical dryer (best)
• Open a valve on the supercritical dryer so it is gently releasing pressure (okay)
• Open a valve on the supercritical dryer so it is more aggressively releasing pressure (good but wasteful)
• Heat the gas tank (works but not well–the gas tank is a much much bigger thermal mass than the supercritical dryer)

And actually, even if you just pressurize the supercritical dryer at room temperature when the gas tank is at room temperature, the expansion of the CO2 entering the vessel as it boils will have a cooling effect and cool your vessel down a little bit, but you likely won’t get enough liquid to siphon over from this effect alone, and certainly doing solvent exchanges won’t work well this way

Carbon Dioxide Facts

Physical State in High Pressure Cylinder: Liquid under own vapor pressure

Fire Potential: Non-Flammable

Major Hazards: Asphyxiant, High Pressure Inhalation

 

Physical Properties of Carbon Dioxide

Formula: CO₂
Molecular Weight: 44.01 g/mol (44.01 lb/lb-mol, or 19.96 kg/lb-mol)
Specific Volume at 70°F and 1 atm (21.1°C 101.325 Pa): 8.74 ft³/lb (0.42 m³/kg)
Specific Heat: 8.92 BTU/(lb mol °F) @ 70°F (21.1°C)
Specific Gravity: 1.555
Gas Density: .1144 lb/ft³ (1.833 kg/m³) @ 70°F 14.7 psia (21.1°C 101.325 Pa)

Psat @ 70°F: 852.8 psia (5.880 MPa)

Liquid Density @ 70°F: 47.64 lb/ft³ (0.763 g/mL)

Boiling Point: Temperature: -109.2°F (-78.4°C)

Critical Point: Temperature: 87.9°F (30.98°C, 304.13 K) Pressure: 1070.6 psia (7.376 MPa)

Triple Point: Temperature: -69.9°F (-56.6°C, 216.55 K) Pressure: 75.13 psia (518 kPa)

Safety Precautions

• Depending on its size, your supercritical dryer may be heavy! Be careful not to drop it and to use two hands if possible to carry it (Note: this applies for the Aerogel.org design; most commercial systems stay put).
• Remember to be gentle and move slowly when moving and tilting the supercritical dryer.
• Always be aware of the status of all three valves of the vessel.
• Always be aware of the pressure and temperature of the vessel.
• Always be aware of the status of the carbon dioxide pressure tank’s valve.
• Be very careful whenever you are venting pressure to the atmosphere. If you begin to feel tired or have difficulty breathing, close any valves open to the atmosphere and get fresh air immediately. This may be a sign that you have displaced too much air with carbon dioxide.
• When liquid carbon dioxide is ejected from the vessel, dry ice may condense. Dry ice is very cold! Be careful when working with dry ice.
• Keep the vessel away from any sources of heat other than controlled hot air from a hair dryer heating during supercritical drying.
• Keep the vessel away from any unnecessary solvents or chemicals.
• Do not store the vessel outside or where it can get wet–this may cause damage to the vessel.
• Do not heat valves or gauges on the supercritical dryer directly.
• When depressurizing using the pressure control valve, stand clear of the pressure stream.
• Work only in a well-ventilated area.
• When opening valves to the atmosphere, be sure to do so slowly. Opening valves fully will release a high-pressure jet of gas and could potentially knock the supercritical dryer over and cause damage to nearby structures and injury to nearby persons.

How to Do Supercritical Drying

Once you have read this page, proceed onward to Supercritical Drying with Liquid Carbon Dioxide Part 2 of 2.