What is Cryogenic Processing

Main Air Compressor (MAC)

The MAC compresses atmospheric air, generally to 60-90 PSIG and delivers it to the system. These compressors are normally driven by electric motors. Interstage coolers are provided to remove heat of compression between each stage of compressor, of which there are normally 2-3.

Front End Clean Up

Modern ASUs utilize a Prepurifier Unit (PPU), which removes moisture, CO2 and most hydrocarbons from the air. Moisture and CO2 must be removed to prevent ice and dry ice from forming later in the process. A PPU is typically made up of a chiller to cool the air to 40-55F, a condensate separator to remove free water and 2 vessels filled with desiccant and mole sieve material, which adsorbs the contaminants while allowing the air to pass through. One bed is always on line to the process, while the other bed is regenerated with heated waste Nitrogen to remove accumulated contaminants. Beds automatically switch every 5-8 hours. The air from the PPU is very close to moisture and CO2 free.

Some older ASUs utilize reversing heat exchangers to accomplish front end clean up. These systems contain special cryogenic heat exchangers that freeze out the moisture and CO2, allowing clean air to flow to the distillation process. The passes in the heat exchanger are switched every 3-10 minutes by a series of butterfly and check valves. One pass removes contaminates while the other is regenerated by outgoing waste gases.

Though moisture and CO2 removal by reversing heat exchangers is economical both in capital cost and operating cost.

Coldbox

The coldbox contains the cryogenic heat exchangers, distillation columns and associated valves and piping. Because parts of this system are very cold, all components are mounted inside the coldbox and then encased in insulation. Coldboxes can be rectangular or cylindrical and are usually tall, some over 200′ depending on capacity and type of Argon system.

Modern coldboxes are filled with perlite insulation, which is light and easy to install and remove, when necessary. Older coldboxes may be tightly packed with cryogenic rockwool, which is hand packed to 14 pounds per cubic foot. It is very time consuming to install and remove.

Expander

All ASUs except some very small units have expanders. Expanders provide the required refrigeration to produce liquids in the distillation column system. Air, Nitrogen or Waste Nitrogen is fed to the expander, causing the wheel to turn and transfer energy to a compressor, generator or oil brake. This transfer of energy causes the gas to cool. As the process continues, the outlet temperature of the expander eventually reaches design temperature while cooling the column system.

Liquid Argon System

There are 2 common types of liquid Argon systems. Many plants don't provide Argon separation equipment at all. In these cases, most Argon simply exits the ASU with the waste gas. The first type utilizes a crude Argon column that concentrates Argon to 2-3% O2 content from a feed from the low pressure column of 88-92% O2. This crude Argon is warmed and mixed with Hydrogen before entering a catalytic reactor, where the H2 and O2 combine to make water. This wet Argon is then dried and again cooled to cryogenic temperatures after which the H2 and N2 are removed in a separator and distillation column, respectively.

Cryogenic Argon systems depend solely on distillation for purification. Since it takes a great number of trays or packing to separate Argon from Oxygen, these columns can be over 200 feet tall. Many new plants use cryogenic Argon systems to avoid using an Argon compressor and Hydrogen in the process. The downside is the long recovery time to achieve purity after a start up or upset to the process, many times over 48 hours.

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