Tomco DE89RB- 1000, Operation & Maintenance Manual

Page: 9 / 87

INTRODUCTION
UNDERSTANDING CARBON DIOXIDE
Carbon dioxide is a chemical compound formed by combining one atom of carbon
with two atoms of oxygen, and is expressed by the molecular formula CO
2
. Carbon
dioxide can exist in any one or all three states of matter: solid, liquid, and/or vapor;
depending on conditions of temperature and pressure.
Under normal atmospheric conditions, carbon dioxide exists as a colorless, odorless
gas which is about 1.5 times heavier than air. Carbon dioxide will not burn or
support combustion and will not sustain life.
When confined within a suitable pressure vessel, carbon dioxide can exist in any of
three states of matter depending on conditions of temperature and pressure. The
point at which all three states may exist in equilibrium is -69.9
o
F (-56.6
o
C) and 60.4
psig (4.2 bar). This is called the triple point. At temperatures and pressures
lower than the triple point, carbon dioxide may be either a solid or a vapor, again
depending on conditions. Dry ice (solid carbon dioxide), at a temperature of -109.3
o
F (-78.5
o
C) at atmospheric pressure, sublimes (transforms directly from solid into
vapor without the formation of a liquid).
The critical point of carbon dioxide is 87.9
o
F (31.1
o
C) and 1070.6 psig (73.8 bar).
At temperatures and pressures greater than 87.9
o
F (31.1
o
C) and 1070.6 psig (73.8
bar), carbon dioxide cannot exist as a liquid. At pressures and temperatures greater
than the critical point, carbon dioxide exists only as a supercritical fluid.
At temperatures and pressures above the triple point and below the critical point,
carbon dioxide liquid with overlying vapor may exist in equilibrium within a closed
vessel. Within this range, there is a definite relationship between temperature,
pressure, and density.
By following the vapor pressure curve in Figure 1.1 on page 1.3, it becomes obvious
that if you desire to store liquid carbon dioxide at 70
o
F (21.1
o
C), the pressure vessel
would have to be built to withstand pressures of around 840 psig (57.9 bar). By
1.2