First off, huge thanks to Exotic Chem Lab for helping me with this video. He's provided me with the valuable information I need to create my own DIY cryocooler. He just started his youtube channel, which you can find at the link below:

   / @exotic_chem_lab  

In this video, I'll be making Ethylene (also called "Ethene", but not to be confused with "Ethane") gas to use as a refrigerant in the second stage of a two-stage vapor-compression system.

Ethylene has the formula C2H4 (as opposed to Ethane, which is C2H6), and has a boiling point of -104C. However, at a pressure of about 16 bar, its boiling point rises to -37C. This means if i compress it to at least 16 bar and then cool it with the evaporator coil from my propane-based vapor compression system, i can force it to liquify, and then expand it in an evaporator to below -100C. This would technically qualify as cryogenic, at least according to US EPA specifications, which considers any temperature below -90C to be "cryogenic".

Typically, a cascade refrigerator would use R508 as its second stage refrigerant, but this substance is no longer produced for environmental reasons, so it's prohibitively expensive. This makes Ethylene the only practical alternative for a hobbyist. Ethylene is also sold as "R1150" refrigerant, but difficult and expensive to acquire in this form. In some countries it may require a license to buy.

Fortunately, DIY production is relatively simple. By boiling ethanol and passing the vapors over an activated alumina (Al2O3) catalyst, the ethanol molecule is dehydrated as follows: C2H6O = C2H4 (gas) + H2O (gas). The output gases are then bubbled through liquid water, and the steam condenses, leaving behind the ethylene gas.

The only catch is that the dehydration has to proceed within a certain temperature range. The optimal temperature for ethylene production is about 450C +/-50C or so. Once the catalyst drops below 350-400C, the majority of the ethanol becomes diethyl ether or (C2H5)2O. Above about 500C, the ethanol molecule starts to decompose entirely, producing pure hydrogen and pure carbon. For this reason, it's necessary to use a thermocouple in the catalyst vessel to ensure optimal temperature is maintained.

Ethylene has a very distinct smell that many people describe as "faintly sweet and musky". To me, the smell is very similar to rotting/overly ripe fruit. It quickly becomes nauseating and can cause dizziness depending on the concentration. A more serious hazard of this gas is that it has almost exactly the same density as air. This means if undisturbed, a cloud of highly flammable ethylene can sit in one spot instead of sinking to the floor like propane or floating up and escaping like methane or hydrogen.

For storage, i collected the ethylene gas in a beach ball then used a fridge compressor to force it into a 2.5 gallon (9.5L) air tank, which had been evacuated, flushed with propane multiple times, then evacuated again to ensure no explosive mixture would occur. This will then be fed into the low-pressure side of the second stage of my vapor compression system when it's built.

Music Used:

Fortaleza - Topher Mohr and Alex Elena
Lobby Time - Kevin MacLeod
Bossa Antigua - Kevin MacLeod