I thought some of you engineers would be interested in reading my web tutorials. I have written some extensive tutorials on the theory and building of a homemade N2 gas generator. It takes regular air, and using pressure swing adsorption, concentrates the N2 up to 99%. This is here. I then use this to supply the N2 for my homemade liquid nitrogen generator. I can liquefy N2 in my garage without any fancy, high-tech equipment. This tutorial is here. What I'd really like to do is fabricate a small turbo expander, but I think I would need a CNC to do it correctly. Anyway, my generator produces about 300-350 cc/hr of LN2.