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CO2 - Liquid or Vapor
In response to several posts regarding whether the CO2 in the high
pressure cylinder is liquid or vapor - It is...BOTH!
At a room temperature of 80 F, the vapor pressure is ~ 880 psia, and the
liquid and vapor phases coexist in the two-phase zone. The proportions
of liquid and vapor will vary according to the total mass contained
within the closed system.
As CO2 is withdrawn from the cylinder, the liquid fraction decreases and
vapor fraction increases. The pressure remains constant matching the
vapor pressure of CO2 at the room temperature..(I have assumed that
cylinder temperature equilibrates with that of the room). When the
last drop of liquid disappears, the system operates in the single phase
zone and the pressure will drop according to the standard gas
relationship (PV=zNRT) . At pressures below 75 psia, the solid- vapor
phase must be considered, but this is outside our discussion. BTW, the
Gibbs Phase Rule is v= c-p-2.
Normally, the CO2 cyclinder should be charged by weight, to ensure that
a vapor pocket is left so that the vapor can disengage from the liquid
before flowing to the regulator. This is important, because the liquid
will produce a much greater refrigeration effect than the vapor when
reduced in pressure (crank up the regulator flow and notice how cold it
gets) - this may result in the regulator valve seat "icing" with
resultant poor performance. This particularly important for CO2 since
the equilibrium at below ~ 70 psia downstream of the regulator is
between the gas and the solid and will make "snow" (which is why this
particularly hard to regulate at low flows & high pressure drops).
The CO2 cylinder should be oriented vertically, but at the low flows the
aquarist uses, a slight tilt is not critical.
One more thing - if you store the cylinder at ~ 100 degrees F (NOT
RECOMMENDED!!!), the cylinder will now be operating at about 1100 psia
in the critical zone where the liquid and vapor properties are