Gas storage and delivery

Question 1

How can compression form a vapour into a liquid?

What makes this more difficult and why?

Answer 1

Compression increases VDW’s forces of attraction which, at a suitable temperature, will exceed kinetic energy and make a liquid.

Higher temps make this more difficult as the average kinetic energy is increased.

Question 2

What happens to pressure if we compress a gas above its critical temperature?

Answer 2

It increases

Question 3

What happens to pressure if we compress a gas below its critical temperature?

Answer 3

It will liquefy and pressure will not increase

Question 4

How do we usually store gasses if their critical temperature is above room temperature?

Answer 4

We normally compress it to a liquid for storage

Question 5

What are isotherms?

Answer 5

Lines that display the effect of temperature and pressure on the physical state of a substance.

Question 6

Describe an isotherm graph?

Answer 6

X = volume
Y = pressure
Different lines at different temperature

Free lines/ no shaded = gas
Liquid phase to top and left
Liquid/vapour phase bottom middle

Question 7

What is the critical temperature of N2O and what does this mean for its isotherm graph?

Answer 7

36.5

Under this temperature it will be vapour, so at greater volumes it is a mix of vapour/gas

As volume decreases and pressure increases (72 bar) it is liquidised.

Question 8

What happens as we decrease the temperature of a gas in terms of pressure required to liquidise it?

Answer 8

Lower pressures are needed

Question 9

What is the triple point of a gas?

Answer 9

Temperature at which a substance can exist as a solid, liquid and gas in equilibrium

Question 10

Kelvin is the thermodynamic triplet point of water. What temperatures does this equate to?

Answer 10

-273.16K
0.01 deg c

Question 11

What is the SVP of water at its triple point?

What is this otherwise called?

Answer 11

4.6mmHg

Sublimation pressure

Question 12

What is the critical temperature of water?

Answer 12

-118.6 deg c

Question 13

How is oxygen stored in a hospital?

Answer 13

Vacuum insulated evaporator

(Diagram on 1.8)

Question 14

What temperature and pressure is an O2 storage vacuum insulated evaporator stored at?

Answer 14

Between -150 and -170 deg c

7-10 bar

Question 15

How does an O2 storage tank work?

Answer 15

They are NOT actively cooled, instead rely on insulation and evaporation to maintain
the low temperature

The small amount of heat energy that enters the VIE → evaporation of liquid
O2 → uses the latent heat of vaporization → ↓ temperature in chamber → maintain cool temperature

If none used - temp increases, therefore pressure increases (3rd law) and O2 is blown off through a safety valve and reduces temp again via latent heat of vaporisation.

O2 vapour leaves the top of the tank to a heat exchanger, it warm and then goes through a pressure regulating valve to the pipeline.

Question 16

In a nitrous cylinder, what component of the contents exerts pressure that is read on the gauge?

What does this mean for identifying how much is left?

Answer 16

Only the vapour

Need to measure the weight of the contents and deduct the tare weight

Can only use the pressure gauge when it’s nearly empty and all the N2O has become gas.

Question 17

How is N2O stored?

Answer 17

Stored as a liquid with vapour above (exerts 44 bar of pressure - vapour always present even when empty so will always exert 44 bar).

Partially filled as liquid is less compressible than gas and needs room to expand if temp changes.

Filling ratio in UK is 0.75 and 0.67 in hotter climates to avoid explosion.

Question 18

What is the filling ratio of a gas stored as a liquid?

Answer 18

The weight of the fluid in relation to the weight of a cylinder full of water.

Question 19

How are CO2 and cyclopropane stored in cylinders?

Answer 19

As liquids

Question 20

How are air and oxygen stored in cylinders and why?

What does this mean for telling how full cylinders are?

Answer 20

As gasses as the CT is too low to store as a liquid.

Means we can use a simple pressure gauge, but a pressure reducing valve needs to be used for delivery to machines or patients.

Question 21

What is unique about the freezing melting point of water in terms of pressure?

Answer 21

The freezing/melting point decreases with increased pressure

Question 22

What happens when entonox is cooled to below its pseudocritical temperature and what is this?

Answer 22

PCT = -7 deg c

Liquefaction (separation or lamination)

Question 23

What pressure are oxygen and air cylinders when full?

Answer 23

137 bar

Question 24

What do cylinders have on them to ensure safety for machinery and patients?

Answer 24

Pressure reducing valves