Cryogenic Engineering
This article is a short introduction to what engineers concerned with industrial plants that operate well below ambient temperature mean by the term. It introduces for the non-specialist some of the terminology and concepts.
Cryogenic – from Greek Kryo (cold) ‘Cold Generation’.
‘Cold’ is a concept used by practitioners and is the antithesis of heat, or ‘heat below ambient temperature’.
They talk about ‘cold production’ by a refrigeration cycle or a turbo-expander or turbine. This extracts energy from the working fluid (usually gas) when the high pressure inlet fluid drives the turbine at high rotational speed speed, leaving at lower pressure and significantly lower temperature by virtue of the energy extracted from driving the turbine wheel and which passes along the turbine shaft.
The energy delivered at the other end of the spinning shaft can be used to drive a compressor or an electricity generator.
Cold is produced in order to carry out processes below ambient. Heat leaks into the process equipment through the insulation and has to be balanced by the cold production for the process to continue working.
Cryogenic processes are typically employed for either or both of –
Separation of light gases by distillation – including those which used to be called the ‘permanent gases’ such as Air, (O2, N2, Ar), CO, CH4 until Pictet and Cailletet produced droplets of liquid air in 1877.
Liquefaction of gases to reduce their volume several hundred fold and enable storage and transport over significant distances – such as CO2, LPG, Ethylene, LNG, N2, O2, H2, He.
The last two gases are particularly difficult to liquefy and this was only first achieved in 1898 and 1908 respectively.
Cryogenic engineering includes the design and construction of processes and equipment to carry out these processes at below ambient temperature.
Cryogenic engineers typically find it more convenient to use the Kelvin temperature scale, where 0 K = absolute zero = -273.15°C. Note: The degree ‘°’ is not used for temperature expressed in ‘Kelvins’.
On the Kelvin scale nitrogen is liquefied at 77.4 K (-195.75°C) and helium at 4.2 K (-268.95°C) at atmospheric pressure (1.01325 bar absolute). At lower temperatures, fractions of a degree become increasingly important!
Equipment for Cryogenic processing
Most cryogenic processes - be they for LNG production, for Air Separation, for CO2 liquefaction for shipping or for Hydrogen or Helium recovery and liquefaction - need common types of equipment. These are :
Special heat exchangers (Brazed compact aluminium plate fin) or spiral wound exchanges to transfer heat between two or more process streams
Refrigeration systems - mechanical vapour compression in one, two or three stages.
Turbo-expanders for near-isentropic work expansion, refrigeration and recovery of energy
Distillation columns for separation
Effective insulation to maintain operation and or store products at the low temperatures described above.
Future articles are planned on these equipment types and also on the above cryogenic processes that use them and what makes an efficient design. (See article on Exergy).