eLFH - Gas Storage Flashcards
Intermolecular forces which overcome kinetic energies to form liquid from gas under appropriate conditions
Van der Waals’ forces of attraction
Increased with pressure at certain temperatures to form liquid from gas
Critical temperature
Temperature above which a gas will not form a liquid no matter how much pressure is applied
Critical pressure
The pressure at which a gas will form a liquid at its critical temperature
Isotherm
Series of lines / graph that describe the the way that temperature and pressure determine the physical state of a substance above and below its critical temperature
Isotherm of N2O at 40 °C
Above Nitrous oxide’s critical temperature
Therefore remains a gas
Inverse relationship between volume and pressure as per Boyle’s law
Isotherm of N2O at 36.5 °C
At its critical temp
Exists as vapour until critical pressure of 72 bar, then becomes liquid
Isotherm of N2O at 20 °C
Below its critical temp so is a vapour
Partly compresses to liquid + vapour at lower pressure of 52 bar as is lower than critical temp so lower pressure required
Horizontal line until inflection point where all vapour has liquified
Fairly characteristic of a nitrous oxide cylinder at room temperature
Triple point
The temperature and pressure at which the solid, liquid and gaseous phases of a substance can exist in equilibrium
The triple point of water is used to define the Kelvin temperature scale
Triple point of water in degrees Celsius
0.01°C
Another name for the saturated vapour pressure at the triple point
Sublimation pressure
I.e Solid can form gas and vice versa
Poynting effect
Molecular interaction
Effect of combining gases (eg Entonox) reduces the critical temperature of the mixture - called pseudocritical temperature
Pseudocritical temperature of Entonox and critical temperatures of N2O and O2
With Entonox:
N2O critical temperature usually 36.5 °C - so below this temp is a vapour / liquid
O2 critical temp is -118 °C so remains as gas
Pseudocritical temp of Entonox / N2O (by Poynting effect) becomes -7 °C at 137 bar
Pseudocritical temp is different at higher or lower pressures
Eg pipeline pressures (4.1 bar) pseudocritical temp is lowered to -30 °C
Lamination (Aka Separation) and how Entonox forms hypoxic mixture
If Entonox cylinder reduced below pseudocritical temperature of -7 °C, N2O can become liquid and separate from oxygen
After liquefaction, nitrous oxide contains ~20% oxygen
Therefore initially breath mostly oxygen gas from cylinder (with little analgesia)
As O2 leaves cylinder, oxygen leaves liquid N2O to establish equilibrium
As cylinder nears empty, then breath vapourised N2O which is now hypoxic mixture (<20% O2 after equilibrium established)
Safety features with Entonox to prevent hypoxic mixture inhalation
Maintain temperature of storage area above 10 °C
Store cylinders horizontally
A dip tube within the cylinder
(Nb: also use of pipeline Entonox as has lower pseudocritical temperature)
Dip tube in Entonox cylinders
Dip tube goes from top to bottom of cylinder
Siphons off any liquid nitrous oxide first
Therefore lowest oxygen concentration ever given to patient is the 20% dissolved in the liquid at the start
Can Entonox cylinders be re-warmed if stored below pseudocritical temperature?
No!
Re-warming does not address the already potentially created hypoxic mixture and does not necessarily re-vaporise the N2O to form the Entonox gas mixture as opposed to separate gases
Why is Oxygen stored in a Vacuum Insulated Evaporator (VIE)
To store O2 below critical temperature of -118.6 °C as a liquid for space saving given amount of O2 used in the hospital
Therefore also needs system to allow gaseous O2 to expand prior to delivery to patient
Diagram of Vacuum Insulated Evaporator (VIE)
Storage of oxygen in VIE
Large oxygen tank outside of hospital
Contains liquid oxygen at -150 to -170 °C and 7-10 Bar
Temperature control of VIE
Thermal insulation by vacuum in the walls of the steel tank
Oxygen usage and vaporisation cools the liquid as latent heat is expended
If no oxygen is used, the temperature rises, increasing the pressure as per third gas law, and therefore O2 “blows off” through the safety valve - this reduced temperature again via latent heat of vaporisation
Oxygen extraction for use from VIE
O2 vapour is channelled from the top of the tank
Goes to heat exchanger
Warmed gaseous O2 goes through series of pressure regulators before joining piped supply
Pressure regulation of VIE
High O2 demand with fast flow rates leads to drop in VIE pressure
Therefore liquid oxygen passes through pressure raising vaporiser and is returned to VIE in gaseous form to restore the pressure
Colour of Nitrous oxide cylinders
Entirely French blue
Storage of N2O in cylinders
Stored as liquid in cylinders
Vapour above liquid exerts gauge pressure of 44 Bar
Liquid less compressible than vapour so cylinders on partially filled
Filling ratio in UK is 0.75
In warmer climates use filling ratio 0.67 to avoid cylinder explosion
Filling ratio
The weight of the fluid in relation to the weight of water if the cylinder were full
Tare weight
The weight of the empty N2O cylinder
Marked on cylinder valve block
Allows measurement of N2O weight within cylinder
Measuring amount of N2O remaining within cylinder
Gauge pressure cannot be used as N2O stored as liquid so saturated vapour pressure always remains at 44 Bar above liquid as is in equilibrium between vapour and liquid
Gauge pressure only changes when cylinder is almost empty and all N2O remaining is in gaseous phase
Therefore have to weigh the remaining N2O fluid (taking tare weight of cylinder into account)
Anaesthetic gases which have been stored in cylinders as liquid
Nitrous oxide
Carbon dioxide
Cyclopropane
Oxygen and air cylinder storage
When cylinders full, O2 and air stored at 137 Bar
As critical temps too low to permit liquid storage in cylinders, gauge pressure is directly related to remaining volume
Require pressure reducing valve or single storage regulator for safe delivery to the anaesthetic machine back bar
