really depends on how complicated you want to go, but to store it under pressure, my design used a gas scrubber system, using sylica gell to dry it, then that was run through an open shaft drive refrigeration compressor to the propane cylinders and stored at 300psi.
the sylica gan be regenerated by heating it up and passing hot air over it, use the color changing type to assist in the opperation, the way to purge the co2 was a simple system of a small bleeder placed at the bottom of the tanks, I'd hold a lighter over the port till it burned then shut the valve off.
2psi compressor off, 10psi compressor on.
Digester > methane co2 seperator via gravity (Old 100 pound propane tank upside down so I could purg the co2 manualy, gass whent in the middle)+ pressure switch 2off 10 on > water bubbler w baking soda mixed in > Coelesent water seperator > Silica dryier (Old flourescent tube filled with it) Compressor inlet > Out let to my tanks + Pressure reliefe & kill switch set to 325psi (Manual reset)
CO2 will settle on the bottom, the methane was taken from the top, gas let in in the center, I used a wide diameter pipe to reduce gas voloucity befor going into the tank. The crude seperator befor the compressor did get rid of the bulk of it but the main tanks still needed manual purging as well. I used all 3/8 stainles pipe as I had the 3/8 fittings on hand and I wanted it able to be switched over to hydrogen easily. the low pressure side was just black poly pipe till it got to the compressor
for the compressor switch I used a ranco low side pressure switch, for the high side a standered HP safety switch, the open drive is the hard part, but I'd be willing to bet with some oil seperators and such you could use a standered hermetic compressor, as they will do 500psi with out flinching. due to the pressure difference you will need a high starting torque type
most importent thing is to clean the gas and dry it befor it ever see's the compressor!
