Lesson 9 - Gases SAQ

** a) What are the principal components in a typical bulk nitrogen storage and supply system? Briefly describe their function.

b) What pressure controls would you find on a typical cryogenic tank, and why are these required?

c) Your manager has complained that the vaporiser on your atmospheric nitrogen vaporiser is always iced up. What is your response?

Response:

a)    The principal components are:

• Storage vessel – insulated vessel to keep the cryogenic liquid at the low temperatures required

• Vaporiser – converts liquid to gas for use in the plant

• Pressure regulation – ensures that the plant is supplied with the correct pressure of nitrogen

b)    Two pressure control loops are normally in place. The first is a pressure relief system which will vent if the pressure in the vessel gets too high. This occurs because even though the tank is well insulated, some heat transfer will still occur which will result in liquid vaporising within the storage vessel. The vaporisation of the liquid ensures that the liquid temperature is maintained. The excess gas is then vented. If the plant demand is high enough, this is not required all the time. The second pressure control loop is a pressure building loop. If liquid is being withdrawn, the volume of liquid drops. The pressure in the vessel will then drop as well. The liquid will vaporise and this will correct the situation, but if it happens too fast for this equilibrium to occur naturally, the liquid can be slightly heated which then aids this process. The vaporisation of the liquid will increase the gas pressure.

c)    An atmospheric vaporiser is designed to convert liquid into gas. In the first part of the vaporiser, the liquid warms up. Because it is at -190°C at the beginning, moisture in the ambient air will freeze onto the surface of the vaporiser. This is normal. The first third of a vaporiser will normally be covered in solid ice. As the liquid vaporises, the gas then undergoes further heating and eventually reaches ambient temperature. The second third of a vaporiser may have some ice, but this tends to melt away quickly due to the ambient air flow. The last third of the vaporiser is normally ice free, as this is typically designed as a safety zone to ensure the gas is at ambient temperature when it is supplied to the brewery. Thus it is impossible to have an atmospheric vaporiser that is not partially iced up.