Supercritical Propane

If Superman were a thermal fluid, he'd be a supercritical one! Supercritical fluids are one of those fascinating things in nature that you never see in everyday life and therefore for which our intuition can just fail miserably. You'd never think that at some point, a fluid would have an identity crisis and a gas would stop being a gas and a liquid would stop being a liquid, and the fluid would decide to mish mash up properties of both into a completely different phase.

I've seen (and done) supercritical CO2 before, but I'd never seen supercritical propane attempted. Propane's critical point is 97 degrees C (206 F) and 42 atmospheres pressure (616 psi). It's a little hotter and a little lower pressure than CO2's critical point, but the temperature is tantalizingly close to the boiling point of water, meaning it's a piece of cake to attain - just drop a sample in a vat of boiling water! The 42 atmospheres pressure is a little trickier - luckily I had sitting around a 1/2" 2000 psi rated ball valve (from ebay) and - even better - a 1750 psi rated sight glass! It's no fun if you can't see it!

I transferred some propane into this contraption by connecting it to a small propane torch tank with an adapter that had 1/8" NPT threads. I opened the propane tank to the contraption and then disconnected and depressurized it to purge a good bit of the air out. I reconnected the tank and then dunked the sight glass end in a cup of ice water, which reduced the pressure on that end and caused propane to flow from the cylinder into the valve/sight glass combo. I let it flow in until the chamber between the ball and the sight glass was about half full of liquid propane. Closing the ball valve trapped the liquid propane on the sight glass side of the valve, and I disconnected the propane tank.

I filled a one liter flask with water and wired up a harness to suspend the ball valve in the water. Heating the water subsequently heated the propane in the valve. Once it hit about 190 F, the propane in the chamber began to boil. As the temperature rose, the liquid fraction appeared to decrease, and finally, about 207 F, it disappeared completely - supercritical! At that point, the vapor and liquid phases decided to settle their differences like Rocky and Apollo, and they should have the same surface tension (none), density, should be miscible with other fluids, et cetera. They become the best of friends, alike in every way. But then Ivan Drago killed... ok this analogy is getting out of hand.

Here it goes! I apologize for the nerdy commentary - actually no, I take that back because this is awesome.

So what's really going on in here? As the pressure and temperature get closer and closer to the critical point, the differences in all properties between the gas and liquid phases start to get smaller and smaller. The density of the liquid phase decreases, while the density of the gas increases. The viscosities likewise approach each other. Then, suddenly, the difference between the two vanishes abruptly. Through the transition, it drags a couple of properties of each. For instance, the surface tension, which is basically what makes a liquid a liquid, disappears, making the fluid gas-like in that sense. The fluid has no tendency to stick together anymore, and is completely miscible (mix-able) in any proportion with any other gas or supercritical fluid. Some liquid-like properties are dragged through too - a supercritical fluid can have strong solvating power, dissolving other substances. These weird properties lead to many of the ways supercritical fluids are used, like in dry cleaning, for their strong solvating power - making them good at getting rid of oily stains on clothes, mixed with their lack of surface tension - making them able to get into all nooks and crannies to dissolve tough-to-get-at stains. Best of both worlds - ain't it great when you can have it all? Sometimes Mother Nature throws us a bone - and looks damn fine doing it in that squeaky clean getup.

Edit: Here's a slightly better video of the transition

DIY Rotovap

I wanted to do some vacuum evaporation projects, so I built a DIY rotovap. This certainly ain't a laboratory grade, strong solvent resistant, low leakage setup, and it has very few speed settings (currently "On" and "Off"), but it does indeed work at allowing vacuum distillations to be carried out, recovering a distinct overhead product and bottoms product.

The evaporation chamber is a Mason jar with a 3/4" x 1/2" PVC bushing epoxied to the lid. I used two electrical conduit locknuts to hold the fitting in place while the epoxy set.

To rotate the evaporation chamber, I mounted an electric motor in a wood frame and mounted a section of 1/2" CPVC pipe in a pillow block bearing. I connected the bearing to the motor shaft with a series of small plastic gears.

I left one section of the CPVC un-cemented on top of the bearing, to allow the pipe to be removed from the bearing. Adding a bit of teflon tape to the pipe before pressing it into the coupling gives a "good enough" vacuum seal.

The heating method for the Mason jar originally gave me trouble. At first, I considered a laboratory hot plate, but was put off by their high cost, and the difficulty I expected to experience in getting uniform heating on the jar. I then realized that I was an idiot, as I already had an electric skillet that I could put a pot of water on to uniformly heat the jar. I tried this, but was less than impressed with the amount of heat I could transfer into the jar. It was also a cumbersome setup. While at the store looking for a smaller, higher heat output hot plate, I realized I was a double mega idiot, and that I had a crock pot that could do exactly what I wanted, with a pretty good heat output to boot. What's more, it's relatively easy to control the temperature in the crock pot over the timescales on which the distillation runs by adjusting the heat setting on the pot.

The next tricky part was the method to give a vacuum-tight seal while allowing continuous rotation. The method I ended up on was to use a compressed air quick connect fitting. These are available at any hardware store at a low cost (about $5 for the male and female components). I used a 1/4" fitting. It does have a rather small restriction through the fitting which isn't ideal for vacuum pressures, but again it works "good enough" for this non-high-vacuum setup. It does have a fair bit of resistance to rotation, which is why the motor I ended up using was so large - a first draft used a smaller motor that couldn't overcome the resistance of the quick connect fitting. The motor I used has a gear reducer that reduces the rotation rate to about 60 rpm, giving it high enough torque to overcome the resistance. It remains to be seen how well the quick connect fitting holds up with use, but they're pretty darn cheap and could be replaced easily.

The overhead business side is the handled by a filter flask. In operation, the flask sits in a bath of ice water to condense the overhead vapors and collect them. This is the only actual piece of lab glassware in the entire setup, and I used it because I had one on hand. I could just as well have modified another Mason jar to the task. A section of PVC extends down nearly to the bottom to force the heated vapors to come into close contact with the cold walls. If the flow didn't contact the vessel walls, a good portion of the overhead product would go straight out the side port to the vacuum pump, which would be bad for the pump and bad for the recovery of the overhead product.

I originally had some difficulty with the sealing element between the filter flask and the PVC, but then fortuitously discovered that a 3/4" PVC compression coupling's rubber gasket is perfectly sized for this filter flask's mouth diameter. A flexible hose connects the filter flask to my Harbor Freight vacuum pump.

Here's a dry run showing everything working together:

Here's a hot run with the first generation (aka dumb) heating method for the boiling jar.

My first run was to distill some tea - I wanted to make some tea concentrate to use in other recipes. It was a technical success, in that I was able to concentrate the tea to a much stronger concentration, with an overhead product that tasted like plain water. The concentrate was decidedly non-tasty, though, so it might not have been a recipe success. I'm not sure if this is because I got the fluid too hot during the distillation - the whole point of vacuum distillation is that you can boil the liquid at a lower than normal temperature. I plan to try again, we shall see if tea-asty product will result!