eLFH - Gas Storage Flashcards

You may prefer our related Brainscape-certified flashcards:
1
Q

Intermolecular forces which overcome kinetic energies to form liquid from gas under appropriate conditions

A

Van der Waals’ forces of attraction

Increased with pressure at certain temperatures to form liquid from gas

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

Critical temperature

A

Temperature above which a gas will not form a liquid no matter how much pressure is applied

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

Critical pressure

A

The pressure at which a gas will form a liquid at its critical temperature

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

Isotherm

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

Isotherm of N2O at 40 °C

A

Above Nitrous oxide’s critical temperature
Therefore remains a gas
Inverse relationship between volume and pressure as per Boyle’s law

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

Isotherm of N2O at 36.5 °C

A

At its critical temp
Exists as vapour until critical pressure of 72 bar, then becomes liquid

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

Isotherm of N2O at 20 °C

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

Triple point

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

Triple point of water in degrees Celsius

A

0.01°C

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

Another name for the saturated vapour pressure at the triple point

A

Sublimation pressure

I.e Solid can form gas and vice versa

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

Poynting effect

A

Molecular interaction

Effect of combining gases (eg Entonox) reduces the critical temperature of the mixture - called pseudocritical temperature

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

Pseudocritical temperature of Entonox and critical temperatures of N2O and O2

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

Lamination (Aka Separation) and how Entonox forms hypoxic mixture

A

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)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

Safety features with Entonox to prevent hypoxic mixture inhalation

A

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)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

Dip tube in Entonox cylinders

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

Can Entonox cylinders be re-warmed if stored below pseudocritical temperature?

A

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

17
Q

Why is Oxygen stored in a Vacuum Insulated Evaporator (VIE)

A

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

18
Q

Diagram of Vacuum Insulated Evaporator (VIE)

A
19
Q

Storage of oxygen in VIE

A

Large oxygen tank outside of hospital

Contains liquid oxygen at -150 to -170 °C and 7-10 Bar

20
Q

Temperature control of VIE

A

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

21
Q

Oxygen extraction for use from VIE

A

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

22
Q

Pressure regulation of VIE

A

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

23
Q

Colour of Nitrous oxide cylinders

A

Entirely French blue

24
Q

Storage of N2O in cylinders

A

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

25
Q

Filling ratio

A

The weight of the fluid in relation to the weight of water if the cylinder were full

26
Q

Tare weight

A

The weight of the empty N2O cylinder
Marked on cylinder valve block

Allows measurement of N2O weight within cylinder

27
Q

Measuring amount of N2O remaining within cylinder

A

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)

28
Q

Anaesthetic gases which have been stored in cylinders as liquid

A

Nitrous oxide
Carbon dioxide
Cyclopropane

29
Q

Oxygen and air cylinder storage

A

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