100 Nano-Stories: HOT vs. COLD!
Episode #12: COLD Supercritical Drying Procedure!
Preface
Hey, Carlos, why is the title called, “HOT v COLD”? Is this some sort of prank?
No, reader! Well, it could sound clickbait-ish, but it is true: there are two types of supercritical drying:
- Low-Temperature Supercritical Drying (LTSCD) (COLD)
- High-Temperature Supercritical Drying (HTSCD) (HOT)
Oh, I see what you mean! But do you think that you can give me more info about what happens in the gel before you go into the process, Carlos?
Definitely! Here is my previous article about this exact question!
On top of that, I will be adding definitions of certain words and extra resources if you are interested to learn more about aerogels, reader!
Tell you what, reader, which way of drying do you want to start with? HOT or COLD?
Let’s pick COLD first, Carlos. Let’s end the article with a warm ending. 😉
LTSCD Drying (COLD) Concepts!
What is LTSCD Drying? LTSCD is the process where the solvent (alcohol-like substance) is replaced with supercritical CO2 that is approaching The Critical Point and is close to ambient temperature.
Uh, Carlos, do you mind if you can explain the letters in bold?
The Critical Point is when the liquid inside the solid structure has both properties of liquid and gas, which is known as a Supercritical Fluid. Once this happens, the molecules of CO2 are now no longer able to pull on each other, so we can fill the pores of the silica gel and depressurize. Ambient temperature is the air temperature of an object/setting/environment where the object (or the gel) is stored.
But what makes COLD Drying special? You still haven’t talked about that, Carlos!
Remember that this is COLD Drying… so that means that we can reach the critical point at such low temperatures! How low of temperatures? The alcohol can be heated up to approximately 40 degrees Celsius (104 degrees Fahrenheit) and 85 bar to achieve the supercritical fluid of CO2. A bar is the metric unit of pressure, and it is equivalent to 100,000 Pascals.
Wow, if that’s how hot- sorry, I mean cold COLD drying is, I wonder how cold HOT Drying is! But what happens next, Carlos?
Now that we covered the essential information, let me explain the process!
LTSCD Drying (COLD) Procedure!
The gel contains a solvent (mostly methanol) and it is placed in an autoclave. We then close the vessel where we added the silica gel so the liquid CO2 can begin to flush out the solvent, and we normally pump the liquid CO2 at around 4–10 degrees Celcius until we reach approximately 100 bars (of pressure).
Sounds good, Carlos! What’s next?
Well, we have to open the valve, or the bars of pressure will be too much for the gel to withstand! Once we reach approximately 100 bars, we open an outlet valve for the solvent (of methanol) to leave after being replaced by the liquid CO2.
Oh, that makes sense, Carlos! But now what?
Once all of our solvents are removed, we close the pump for the CO2, and we begin to raise the temperature to about 40 degrees Celcius! This is about 9 degrees above the critical temperature of CO2 (35 degrees Celcius!) However, the pressure is going to remain at around 100 bar to ensure we reach The Critical Point.
So it sounds like this is one of the important steps of the process!
It sure is! But we are not over with the procedure! Once we reach The Critical Point and the CO2 is now a Supercritical Fluid, we begin to depressurize the chamber under what is known as a natural flow.
Okay… but how do you remove the gel?
We first have to reach ambient pressure, reader. Ambient Pressure is the air pressure of an object/setting/environment where the object (or the gel) is stored. Once the gel is depressurized to the ambient temperature, the entire system/autoclave is cooled down to about room temperature, which is about 20 degrees Celcius (68 degrees Fahrenheit).
Hey, ambient pressure sounds similar to ambient temperature, Carlos!
Good! You are paying attention, reader! And guess what? Once we reach room temperature, we end up with this:
Amazing! That aerogel looks stunning! But is that all you are going to talk about, Carlos?
Absolutely NOT! The journey to COLD Drying has just begun!
COLD Drying Explained Further!
Remember when I was talking about how we should remove methanol and liquid CO2? Let’s dive more into that for the remaining minutes that we have!
Instead of heating and cooling liquid CO2, we are now using supercritical CO2 to get rid of the steps involving heating & cooling the solvent & liquid CO2, not to mention we are now able to reuse & recycle the CO2 after every use of COLD supercritical drying.
That’s nice! Good job to them for recycling liquids!
But the drying step in supercritical drying is dependent on the diffusion of our supercritical CO2 in the solvent of our silica gel. Diffusion is when a substance spreads out wider than its original state of being packed tightly in one place.
Well, that is peculiar… but how does the liquid CO2 move around in the gel?
It is caused by one of two transport mechanisms are Knudsen Diffusion and Surface Diffusion.
Knudsen Diffusion is the least common mechanism for the transport rate of the solvent & liquid CO2. But it only works if the pore diameter is smaller than the path the molecule is going in the silica framework. The heavier a molecule is in our solvent, the molecular velocity will decrease.
Surface Diffusion is the more common mechanism for the transport rate of our solvent & liquid CO2. This motion mostly revolves around increasing the temperature, something that we use during Supercritical Drying.
Okay, Carlos, I think let’s stop here! You can talk more about this another time, but you better talk about HOT Drying in your next article!
Well, sure! Maybe in the next article, I can talk about the Aerogel Jacket! See you then! 👋🏽
Vocabulary 📓
Solvent - The liquid in which a solute is dissolved to form a solution.
Ambient Temperature - The air temperature of an object/setting/environment where the object (or the gel) is stored.
Bar - The metric unit of pressure; equivalent to 100,000 Pascals.
Pascal - The SI Unit of Pressure, and it is equal to one kilogram per meter per second squared.
Autoclave - A machine designated to increase temperature & pressure in elevated temperature and pressure to the ambient pressure and temperature.
Critical Point - The pressure-temperature curve that designates conditions under which a liquid and its vapor allow for properties of both liquid and gas to coexist.
Diffusion - A substance spreads out wider than its original state of being packed tightly in one place.
Ambient Pressure - The air pressure of an object/setting/environment where the object (or the gel) is stored.
CO2 - Carbon Dioxide.
Supercritical Drying - Liquid in a gel/substance is being transformed into a gas with the absence of the surface tension and the capillary stress of the gel. It’s what allows for a silica gel to transform into an aerogel.
Resources + Bonus Content 💻
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