Liquid CO2 can be stored indefinitely at high pressure or Low pressure as follows:
High Pressure CO2
High pressure liquid CO2 is produced by compressing the gaseous CO2 in multistage compressors to pressures in the neighbourhood of 69 bar (1000,76 psi) pressure, then cooling it to around 18 °C (64,4 °F). It is customarily filled into specially constructed steel cylinders. Like water, liquid CO2 can be weighed, and this is the normal form of measuring it.
Low Pressure CO2
Low pressure liquid is an alternative method of storing CO2 and is produced either by expanding high pressure CO2 to a lower pressure or by refrigeration. It is held in specially constructed storage tanks, heavily insulated and equipped with refrigeration units to hold the internal tank pressure at or below 21 bar (304,58 psi) and -18 °C (-0.4 °F) temperature. Pressure switches are incorporated to control the refrigeration units and safety mechanism and electrical alarms are fitted to release the tank pressure and raise an alarm in the event of refrigeration malfunction.
Low pressure liquid CO2 (sometimes referred to as “bulk”) is transported to one place from another in insulated road tankers or trailers (or, in some countries, by rail tank wagons), the liquid being simply transferred from mobile to static tank by pumping or gravity feed.
Unlike water, carbon dioxide cannot exist in the open air in liquid form. It must be held under pressure or refrigeration (or a combination of both) to remain in the liquid state.