Gas Storage

Question 1

What is the difference between a gas and a vapour?

Answer 1

A gas is above the critical temperature therefore cannot be liquefied by pressure alone.

A vapour is a gas below its critical temperature meaning that it can be liquefied by pressure alone.

Question 2

Chemically explain the critical temperature?

Answer 2

The critical temperature of a substance is the temperature above which it cannot be liquefied, however much pressure is applied.

The critical temperature is to do with intermolecular (van der vaals) forces.

As a gas is compressed the van der vaals forces of attraction increase and eventually overcome the kinetic energy and dispersive forces causing the gas to liquefy.

As the temperature increases the kinetic energy and dispersive forces increase. The critical temperature is the point at which the kinetic energy and dispersive forces are too great for the gas to liquefy (regardless of the pressure).

Question 3

What is the critical pressure?

Answer 3

The critical pressure is the vapour pressure of a substance at its critical temperature.

Question 4

What is the critical temperature and pressure of oxygen?

Answer 4

Critical temperature -118 degrees celsius
Critical pressure 50 Bar

Question 5

What is the critical temperature and pressure of nitrous oxide?

Answer 5

Critical temperature 36.5 degrees celsius
Critical pressure 72 Bar

Question 6

What is the critical temperature and pressure of carbon dioxide?

Answer 6

Critical temperature 30 degrees celsius
Critical pressure 73 Bar

Question 7

What is an isotherm?

Answer 7

Isotherms are a series of lines that describe the way in which temperature and pressure determine the physical state of a substance above and below the critical temperature.

X axis volume
Y axis pressure

Question 8

Describe the isotherms of nitrous oxide at 40 degrees?

Answer 8

Temperature 40°C: Nitrous oxide is above its critical temperature and exists as a gas. The inverse relationship between temperature and volume follows a hyperbolic curve which is explained by Boyle’s Law. Therefore as you reduce the volume the pressure increases.

Question 9

Describe the isotherms of nitrous oxide at 36.5 degrees?

Answer 9

Nitrous oxide is at its critical temperature. It exists as a vapour at low pressure and then liquefies at the critical pressure of 72 bar. Liquids are relatively incompressible, explaining the inflexion point in the curve and steep pressure rise for any reduction in volume.

Linear curve with sharp inflexion at 72 bar.

https://thelaymedicalman.com/2018/06/25/nitrous-oxide-isotherm/

Question 10

Describe the isotherms of nitrous oxide at 20 degrees?

Answer 10

Nitrous oxide is below its critical temperature. At 20°C the vapour partly compresses to a liquid with only 52 bar. Further reduction in volume causes more vapour to condense with no change in pressure. A horizontal line is seen before the inflexion point at which all vapour has become liquid. This is characteristic of a nitrous oxide cylinder at room temperature.

Question 11

What is a triple point?

Answer 11

The triple point is the temperature and pressure at which the solid, liquid and gaseous phases of a substance can exist in equilibrium. Fixed points such as this are essential in establishing a temperature scale.

Kelvin is defined by the triple point of water (0.01degrees C)

Question 12

How is entonox made?

Answer 12

Via the Poynting effect.

The Poynting effect involves the dissolution of gaseous O2 when bubbled through liquid N2O, with vaporisation of the liquid to form a gaseous O2/N2O mixture.

Question 13

What is the pseudocritical temperature of entonox?

Answer 13

-7 degrees celsius

Question 14

What would happen if a cylinder of entonox was stored at less than -7degrees?

Answer 14

If it was stored at less than -7 degrees C (i.e below its pseudocritical temperatures) the N2O would start to liquefy and separate from the O2.

This process is called lamination or separation.

After liquefication the nitrous oxide will have approximately 20% O2 dissolved in it.

Question 15

What are the potential risks of liquefication of entonox?

Answer 15

If liquefication occurs there is a risk that initially you will just be giving the O2 in the cylinder therefore providing no analgesia.

As the O2 is used up you then risk giving a hypoxic mixture of vaporised N2O

Question 16

What safety measures can be used to avert the risk of liquefication of entonox?

Answer 16

Ensuring cylinders are stored at correct temp (above pseudocritical temperature)

Storing cylinders horizontally.

Utilising a dip tube. A dip tube reaches to the base of the cylinder and siphons off the liquid nitrous oxide first, ensuring that the lowest concentration of oxygen ever given to a patient is the 20% dissolved within the liquid from the start.

Question 17

How is oxygen stored in a hospital?

Answer 17

It is stored as liquid oxygen in vacuum insulated evaporators. This saves space and is more economical. Oxygen is piped from these VIE’s to the hospital.

Cylinders of gaseous oxygen are also available.

Question 18

How does a vacuum insulated evaporator work?

Answer 18

Liquid oxygen is held in an insulated steel tank at -150 to -170 degrees C.

As oxygen vaporises latent heat is expended and cooling the liquid O2.

If no O2 is vaporised then the temperature rises, as the temperature rises the pressure rises.

There is a blow off valve set at 1500KPa, if the pressure rises above this O2 passes through the valve. The blow off reduces heat by the latent heat of vaporisation.

If there is heavy demand for oxygen, with fast flow rates, the pressure in the VIE drops. If this occurs, liquid oxygen passes though the pressure raising vaporizer and is returned to the VIE in gaseous form to restore the pressure within.

Vaporised O2 passes through a heat exchanger and pressure regulators before it is piped into the hospital.