You don't need liquid nitrogen. In fact you don't even have to freeze the material first. The vacuum itself causes it to freeze due to the heat loss caused by the water evaporation. Apparently prefreezing does have benefits though. If you let it freeze just by the vacuum there can be frothing (see page). You do need some slight heating to avoid it getting too cold which would interfere with the drying. I don't know what temperature would be best to maintain it at, probably a little below 0 C. This does seem to be the premium method for drying though. A 12 or even 24 hour drying time would be pretty sweet, and having buds that look almost exactly like they did when they were picked. Could be a little bulky though, for handling. Oddly enough, when you search "Cannabis freeze drier" you get nothing but Harvestright, $3000. If you go to Best Value Vacs you can buy vacuum chambers fairly cheap though. That would be a start for making a DIY unit. Just need a vacuum pump and a water trap. The chambers are a little small though, if you want to dry much product.