Air separation is the most common process used to extract one or all of the main constituents of atmospheric air. The three main components are Nitrogen (78.1%), Oxygen (20.9%) and Argon (.9%). The remaining gases in the air are in trace amounts and normally not recovered. In very large air separation units (ASU) Neon, Xenon and Krypton are recovered in small amounts.
Cryogenic air separation utilizes the differing condensing/boiling points of the components of air to enable separation by distillation at cryogenic temperatures.
At atmospheric pressure the main components of air have the following condensing/boiling points:
Nitrogen -320.4° F
Oxygen -297.3° F
Argon -302.5° F
Since air is a simple mixture, liquefying and distilling air provides a process to successfully separate the Nitrogen, Oxygen, and Argon. All ASU’s use this process.
Basic Air Separation Unit (ASU) Components
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.
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.
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.
Different Types of Air Separation Units (ASU)
All gaseous product
On site gas generators are by far the most common type of ASU. These plants can produce Oxygen only, Oxygen and Nitrogen or Oxygen, Nitrogen and Argon or just Nitrogen. In all cases the gases that are not fully purified are used for refrigeration and vented to atmosphere. These units separate the air in liquid form, but utilize the refrigeration from the liquids before they exit the coldbox. The products come off the coldbox at fairly low pressure and ambient temperature. Product compressors are then used to increase the products to required header pressure. In some processes, liquid products are pumped through the main heat exchangers where they are warmed to ambient temperature at the required pressure. Gas generators typically produce between 10-2000 short tons of combined product.
All Liquid Product
This is normally considered a merchant plant. All desired product is liquefied for shipment in cryogenic transport trailers or rail cars. Generally, these units make liquid Oxygen (LOX), liquid Nitrogen (LIN) and liquid Argon (LAR), though in some oil/gas production areas there are large liquid Nitrogen only ASUs. These products are delivered into cryogenic tanks at the users site, where it is either warmed back to a gas before use or used as a liquid. Typically the only users that utilize liquid products are food freezers, oil field service companies or other processes that require very cold temperatures.
Liquid plants are typically sized to produce from 150 to 1000 short tons per day of combined product. These ASU’s have an additional section of equipment called a Nitrogen Liquefaction Unit (NLU) to provide the necessary refrigeration to liquefy all products. The NLU is generally the largest power user by far in an ASU.
Combined Liquid and Gas Product
Commonly called “piggyback plants,” these ASUs produce mostly gaseous product for pipeline use, but also have NLU’s to produce liquid products for shipment off site.
Piggyback plants are generally sized from 300 to 1000 short tons combined product.
For more information, contact Ranch Cryogenics, Inc today