Cryogenic Refrigeration Part II - Lower temperature refrigeration.

Thermodynamics and terminology

To liquefy what used to be called the ‘permanent gases’ needs much lower temperatures than can be readily achieved with a vapour compression cycle.  Therefore, a different approach is required.  A reminder of the 1st and 2nd laws of Thermodynamics may be appropriate – expressed in simple terms.

Thermodynamics refers relationships between forms of energy of which heat is one important form.

1st Law: Energy can be transferred but the total amount of energy remains constant.

2nd Law: Heat energy always ‘travels’ ‘downhill’ from a higher temperature to a lower temperature. Without a difference in temperature nothing happens. In practice, the flow of heat from a warm body to a cooler one produces something called Entropy - of which more later.

Also, at this point it is convenient to introduce the Kelvin temperature scale, which makes it easier to understand cryogenic processes without having to deal with negative temperature values.

Absolute zero is  -273.15C or zero degrees Kelvin. The size of one degree is the same on both scales. Zero degrees Celsius equals 273.15 degrees Kelvin.

A common factor in this different refrigeration approach is to feed a gas – which may be the gas to be liquefied or another fluid into the process at high pressure.  Unless it is available at high pressure, this gas needs pre-compression at ambient temperature with inter- and after-cooling using air or cooling water. The energy for this compression is significant.  The high pressure of the feed gas is often the main source of energy that drives and enables the process to work.

The feed gas typically has to be dehydrated to prevent freeze out in subsequent low temperature steps.  In the case of air, CO2 must also be removed by adsorption.  This purification or pre-treatment step is normally done after or partway through feed gas compression.

The treated HP gas is then cooled down counter-current with returning cold and lower pressure gaseous product stream or streams which are thereby warmed back up to ambient.  The heat exchanger operation must be ‘in balance’ (1st Law) and the warmer stream must be at a higher temperature than the cooler stream at all points in the heat exchanger (2nd Law).

Examples of Cryogenic processes.

There are two principal types of industrial cryogenic processes, namely Separation processes and Liquefaction processes.
A 3rd type produces refrigeration continuously to support some other process that occurs at low temperatures.  An example could be very low temperature superconducting electromagnets used in nuclear fusion research. MRI machines also have high powered superconducting electromagnets generally cooled by liquid helium.

Hybrid processes also exist such as an air separation unit (ASU) which splits air into its constituents – nitrogen, argon and oxygen by distillation in up to three or four cryogenic distillation columns and may deliver one or more of the products as a cryogenic liquid and the others as ambient temperature gas.

Other process such as LNG produce only liquid methane – plus a small stream of light ends including N2 which is used as fuel.

A typical Double-Column Air Separation (ASU) process.

Conversely, the processes to separate semi-pure hydrogen from a mixture with light hydrocarbons or from Ammonia synthesis purge gas containing N2, Ar and CH4 operate at very low temperature.  But they deliver the H2 product and the other rejected components as ambient temperature gases.

Cryogenic recovery of Hydrogen from Ammonia plant purge gas.

To produce H2 as a liquid  would require cooling to about 20 K which is not possible without the other components freezing out and blocking the process.  Final purification of hydrogen by cryogenic means is difficult - if not impossible and the ambient temperature Pressure Swing Adsorption process is used.

It may of interest to mention that both the hydrogen-nitrogen and helium-nitrogen binary mixtures have two bubble points and there is a wide range of compositions for which no liquid bubblepoint exists.

Both the illustrated processes such ASU and Hydrogen recovery together with a host of others such as Nitrogen Rejection from natural gas (NRU) make use of :
a) the Joule-Thomson (J-T) effect in which a gas cools when it is expanded through a valve and
b) work expansion where a gas cools much more than J-T because work or energy is extracted from the expanding gas.

These are developed in more detail in part III and IV.