5. Supply of Medical Gases Flashcards
How is oxygen manufactured?
> The most common method of
manufacturing oxygen commercially
is by the fractional distillation
of liquefied air.
This method produces oxygen which is over 99% pure.
> Alternatively,
oxygen concentrators
containing zeolite adsorbents
can be used.
Zeolite selectively adsorbs
nitrogen and so
delivers oxygen that is 90–93% pure.
The major contaminant is argon.
Oxygen concentrators are commonly used in aircraft, submarines, military field hospitals and at home.
How is oxygen stored?
- VIE
- J cylinders
- Anaes machine E
Hospitla back up oxygen supply
> The hospital back-up oxygen
supply comes from a cylinder manifold
(size J cylinders arranged in series),
which stores oxygen as a
compressed gas
at room temperature.
The oxygen from these sites get carried
to the hospital in pipelines coloured
white delivered at a pressure
of 4 bar (400 kPa).
Anaesthetic machine oxygen store
> Oxygen on the anaesthetic machine is stored as a compressed gas in molybdenum steel cylinders (size E cylinders) with black bodies and white shoulders at a pressure of 137 bar (13 700 kPa).
Main hospital o2 supply
What is it
How much can this hold
How is it stored
(+ what conditions)
How does this differ - why is this relevant
> The main hospital supply of oxygen
comes from a vacuum-insulated evaporator (VIE),
which holds up to 1500 L of liquid oxygen.
This is the most economical
and
space-saving way of storing oxygen.
The liquid oxygen is stored
at a temperature between
-150 and -170 °C
(below its critical temperature of -119 °C)
and at a pressure of 7 bar
(this is the saturated vapour pressure
(SVP) of oxygen at its stored temperature).
Because it is in liquid form,
oxygen in a VIE behaves
like nitrous oxide in a cylinder
and therefore in order to
know how much oxygen is remaining,
the storage vessel rests on a
weighing balance so that the
mass of liquid oxygen can be measured.
Why are oxygen cylinders filled to 137 bar?
Cylinder technology is old
and
filling pressures of compressed gases
were originally measured in
pounds per square inch (psi).
The cylinders were filled to 2000 psi,
which is equivalent to 137 bar.
How do you calculate the volume of oxygen that can be
discharged from a 10 L cylinder?
Oxygen in a cylinder is stored
as a compressed gas
and obeys the ideal gas
law (P1V1/T1 = P2V2/T2),
which is used to calculate the volume of O2 available:
> Cylinder capacity is fixed = 10 L [V1]
> Gauge pressure of cylinder = 13 700 kPa
> Absolute pressure of cylinder =
gauge pressure (13 700 kPa) +
atmospheric pressure (100 kPa)
= 13 800 kPa [P1]
> Volume of O2 available = [V2]
> Pressure of atmosphere = 100 kPa [P2]
As temperature is constant, T1 = T2 and hence equation is further
simplified
P1 × V1 = P2 × V2
→ 13 800 × 10 = 100 × V2
→ V2 = 1380 L
> But 10 L will always remain in the cylinder, and hence 1370 L of O2 is available.
Draw a graph showing what would happen to the gauge pressure of an oxygen cylinder over time if it were being used continuously at the same rate
161
How is nitrous oxide manufactured?
Nitrous oxide (N2O) is manufactured by the thermal decomposition of ammonium nitrate.
How is nitrous oxide stored?
The critical temperature of nitrous oxide is
36.5 °C
and therefore at room temperature
N2O exists as a liquid with its vapour.
> On the anaesthetic machine,
N2O is stored as a liquid in
molybdenum steel cylinders
with blue bodies and blue shoulders
at a pressure of 52 bar
(this is the SVP of N2O vapour above its liquid).
> N2O cylinders are only
partially filled because liquids
are less compressible than gases
and
should these cylinders be subjected to
temperatures above its critical temperature
(e.g. in a desert or during a fire)
all the N2O would convert to a gas,
causing a massive explosion.
Therefore,
depending on the temperature of the country,
N2O cylinders have different filling ratios.
> The main hospital supply of N2O comes from a cylinder manifold where once again the N2O is stored as a liquid at room temperature.
Why do different textbooks quote varying gauge pressures for N2O cylinders?
N2O cylinder gauge pressures
reflect the SVP of the N2O
above its liquid and are quoted from
between 44 and 54 bar
depending on the temperature at
which the measurement was taken
(as SVP varies with temperature
The filling ratio =
Weight of substance cylinder is routinely filled with
_______________________________________
Weight of water cylinder could hold if full
> In tropical countries the filling ratio is 0.67 but in temperate climates it is 0.75.
How can you calculate the volume of N2O that can be
discharged from a cylinder?
Because these cylinders contain
liquid and vapour
you cannot apply the
ideal gas law.
Instead you need to weigh the cylinder in order to work out the weight of the remaining N2O and then apply Avogadro’s hypothesis:
Weight of N2O No. of moles of N2O in cylinder =
\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_
Mol weight of N2O
= (Cylinder weight – Tare weight)
________________________
44
Volume of N2O available (L) =
No. of moles × 22.4 L
Avogadro’s hypothesis states that at standard temperature and pressure
1 mole of gas occupies 22.4 L
Draw a graph showing what would happen to the gauge pressure of a nitrous oxide cylinder over time if it were being used continuously at the same rate
Fig. 54.2 Change in gauge pressure over time for a N2O cylinder
Linear out from mid of y then
Sharp decline in pressure
> Initially,
the N2O cylinder contains
both liquid and vapour.
> As the cylinder empties,
the vapour is used up
and the liquid continues to vaporise
until it is all used up.
> This explains the
constant cylinder gauge pressure
until all the liquid has been used up, at which point the cylinder only contains gas and behaves like the oxygen cylinder and obeys Boyle’s law, with pressure declining over time with use.
> This is actually a simplification;
in reality the initial pressure is not perfectly
constant because as the N2O vaporises
from the liquid phase it cools slightly
(latent heat of vaporisation),
and therefore gauge pressure does fall slightly.
Define the following terms
. > Gas
– substance which is
above its critical temperature.
A gas will expand to fill any space available.
