Can we mix liquid CO2 with compressed CO2 gas cartridge in brine solution in a vessel?
YES IT WILL BE DANGEROUS to inject liquid CO2 at RT and heat, at least you risk blowing the rupture disc [I hope you have one !]. You could get a pressure much higher than intended. You could add a calculated amount of CO2 at RT and heat slowly while watching the pressure very closely. You may need to do several experiments to get the right amount of CO2. The cylinder will only give you the vapor pressure of CO2 at RT – DO NOT HEAT IT OR YOU WILL BLOW THE RUPTURE DISC. Be warned that the critical temp of CO2 is 31 C so it will be exceedingly nonideal around RT. So use actual data, NOT the ideal gas law. See http://webbook.nist.gov/cgi/fluid.cgi?T=… But note that the vapor pressure will be depressed by the presence of water.
