Universal Industrial Gases, Inc...Air Separation Technology - Producing Oxygen, Nitrogen, Argon

Air separation plants produce one or both of the two most common atmospheric industrial gases (nitrogen and oxygen) as gases and can also co-produce liquid products. Some air separation units also produce compressed dry air, argon, ultra high purity (UHP) oxygen, or, occasionally, the "rare gases" (neon, krypton and xenon).

Air separation plant products may be delivered to customers as gases through local pipelines or regional networks; or they may be delivered as liquids in "bulk liquid" trailers, or as compressed gases in high-pressure cylinders.

There are many potential variations in process and mechanical design details which reflect application-specific details of the desired mix of gas and liquid products, required product purities, delivery pressure and production rates.

Air separation plants employ one of two basic types of process technology:


	Cryogenic air separation plants - Produce nitrogen, oxygen and argon as gas (and liquid) products using very low temperature cryogenic distillation to separate and purify the desired products. Cryogenic plants are most commonly used to produce high purity products at medium to high production rates. They can produce products as gases or as liquids.

	Non-cryogenic plants - Produce gaseous nitrogen or oxygen products using near-ambient-temperature separation processes. Non-cryogenic separation processes are most commonly used when high purify nitrogen or oxygen is not needed (e.g. to produce nitrogen which is 98 to 99.5% oxygen-free, or oxygen at about 93% purity) and when product demand is relatively low. There are two major types of non-cryogenic processes: selective adsorption or differential permutation through membranes to produce relatively pure oxygen or nitrogen. These air separation processes use differences in properties such as molecular structure, size and mass to achieve the desired degree of product purity.

This page provides an introduction to the characteristics of cryogenic and non-cryogenic plants and discusses some of the issues to be considered when defining an optimal supply system.

Other pages in our Air Separation Technology and Distribution Optimization section provide additional information on:

Various types of air separation plants
Inter-relationships between plant type and delivery mode, and
Other factors which must be addressed when defining an optimal supply system for a site.

All air separation processes start with compression, drying and purification of air. In some cycles, multiple compressors may be used to boost the pressure of the nitrogen and / or oxygen products leaving the separation and purification process. Additional compression (followed by expansion) may be included to produce large amounts of refrigeration to deliver some portion of the air separation unit (ASU) products in liquid form.

The cost of electricity is the largest single operating cost incurred in air separation plants. It is usually between one third and two thirds of the operating costs associated with producing gas and liquid products. Electric motors are used to drive the compression equipment, and power is required for process heaters, instrumentation systems and cooling systems. Electrical power is just as much a raw material as air when manufacturing atmospheric industrial gas products.

Small gaseous product plants may use hundreds of kilowatts (kW). Large liquid plants may have power demands measured in thousands and tens of thousands of kilowatts (megawatts or MW).

The various separation technologies that produce commercially useful products from air are based on differences in boiling points (cryogenic distillation); or on differences in molecular weights, molecular size and other properties (non-cryogenic separation processes)

Non-cryogenic plants are less energy efficient than cryogenic plants (for comparable product purity) but may cost less to build, in particular when the required production rate is relatively small. Non-cryogenic processes are most suitable when high purity product is not required by end-use applications. This is because the physical size of the plant can be reduced as required purity is reduced, and the power required to operate the unit is reduced as well. Non-cryogenic plants are relatively quick and easy to start up, which is useful when product is not needed full time.

Non-cryogenic processes employ membranes or adsorbents (PSA/ VPSA) to remove the unwanted components of air. They produce oxygen which is typically 90 to 95% pure, or nitrogen which is typically 95 to 99.5% oxygen-free.

At high production rates, cryogenic processes are the most cost-effective choice. Cryogenic processes can produce very pure end products; and must be used to produce liquid nitrogen, oxygen and argon.

Within each technology family - cryogenic and non-cryogenic - there are numerous choices which can be made regarding the specifics of process design, machinery configuration and system control. These all have capital cost / energy cost / and operational flexibility / plant reliability tradeoffs.

Gas	% by Volume	% by Weight	Parts per Million (by Volume)	Chemical Symbol
Nitrogen	78.08	75.47	780790	N₂
Oxygen	20.95	23.20	209445	O₂
Argon	0.93	1.28	9339	Ar
Carbon Dioxide	0.040	0.062	401	CO₂
Neon	0.0018	0.0012	18.21	Ne
Helium	0.0005	0.00007	5.24	He
Krypton	0.0001	0.0003	1.14	Kr
Hydrogen	0.00005	Negligible	0.50	H₂
Xenon	8.7 x 10^-6	0.00004	0.087	Xe

Cryogenic air separation processes rely on differences in boiling points to separate and purify products. The basic process was commercialized early in the 20^th century. Since then, numerous process configuration variations have emerged, driven by the desire to produce particular gas products and product mixes as efficiently as possible at various required levels of purity and pressure. These air separation process cycles have evolved in parallel with advances in compression machinery, heat exchangers, distillation technology and gas expander technology.

Lin Assist plants provide high purity at lower capacities

New plants designed and built by UIG incorporate the latest technologies - such as packed columns, argon purification by distillation, and sophisticated control systems.

A nitrogen-only production plant operated by UCG.

Cryogenic nitrogen plant serving a specialty chemicals maker.

UIG designed all-new air separation plant and with 350 tpd liquefier supplied to Airgas New Carlisle Indiana and Carrollton Kentucky

All cryogenic processes include these steps:

Filtering and compressing air
Removing contaminants, including water vapor and carbon dioxide (which would freeze in the process)
Cooling the air to very low temperature through heat exchange and refrigeration processes
Distilling the partially-condensed air (at about -300˚F / -185˚C) to produce desired products
Warming gaseous products and waste streams in heat exchangers that also cool the incoming air stream

Cryogenic processes are the most cost effective separation process for producing at high production rates and are capable of making the highest purity products (see System Optimization Guidelines).

The portions of the cryogenic air separation process that operate at very low temperatures (the distillation columns, heat exchangers and cold interconnecting piping sections) must be well insulated to minimize energy consumption and avoid operating problems. To accomplish this, these components are located inside insulated, sealed (and nitrogen purged) structures called "cold boxes”. Cold boxes may have a rectangular or round footprint and, depending on plant type and capacity, may measure approximately 2 to 4 meters on each side and have a height of 15 to 60 meters.

In a classic cryogenic air separation unit the oxygen and nitrogen products are produced at close to ambient air temperature and at relatively low pressure. (Depending upon product mix and manufacturer/ operator preferences, cryogenic air separation plants may be referred to as an Air Separation Unit, ASU, Oxygen Plant, Oxygen Generator, Nitrogen Plant or Nitrogen Generator.) However, modifications to the basic cycle allow products to be produced at higher pressures. The modification may be as simple as adding product compression, or the air separation process technology (complexity and configuration) may be chosen to allow one or more products to be produced at higher pressure. Optimal process selections allow for the most economical tradeoff between capital cost, power consumption, and overall system operating simplicity and reliability.

Nitrogen plants are routinely designed for delivery pressures of either 1-3 atmospheres or 6-8 atmospheres). Oxygen plants can incorporate what are known as pumped LOX cycles, which produce products at pressures of 7 barg (100 psig) to several hundred psig without product compression. These cycles use additional feed air compression and a special LOX vaporization / feed air cooling heat exchanger system to eliminate product oxygen compression.

Most cryogenic air separation plants can produce a few percent of the plant product as liquid. If large amounts of oxygen or nitrogen are desired in liquid form, additional refrigeration (external to the basic air separation unit) is required. The set of equipment which accomplishes this task is commonly called a Liquefier, and it uses nitrogen as the primary working fluid. Through compression, expansion and heat exchange, liquefiers produce the refrigeration needed to lower the temperature of large amounts of product from near-ambient to about -300˚F / -185˚C, then condense and sub-cool those products prior to sending them to storage. Sub-cooling is desirable to minimize vaporization of stored product due to heat leak into the storage tank.

Low usage nitrogen customers can be most economically served with nitrogen PSA systems employing single-bed technology

Most non-cryogenic nitrogen PSA units employ dual-bed technology.

Example of a gaseous oxygen plant employing non-cryogenic oxygen PSA technology, relocated by UIG

Several types of non-cryogenic air separation processes can be used to produce nitrogen or oxygen as gaseous products only.

Non-cryogenic air separation processes are most likely to be a suitable and cost effective choice when high purity product is not required and/or when the required production rate is relatively small. They may also be advantageous at higher production rates when product usage is not continuous, since they can be started up quickly and shut down when product is not needed. A buffer tank is commonly used downstream of the production unit to smoothly accommodate varying demand patterns.

Non-cryogenic processes use physical properties other than boiling point to separate and purify components of air at close-to-ambient temperature. Systems belong to one of two major technology categories: adsorption processes and membrane diffusion-separation systems. Adsorption-based processes may be described using a number of generic names (Pressure Swing Adsorption or PSA, Vacuum Swing Adsorption or VSA, Vacuum-Pressure Swing Adsorption or VPSA) or by trade names. The same holds true for Membrane separation systems.

PSA, VSA and VPSA systems use differences in adsorption of gases on specially-fabricated materials to make the desired separations. Different adsorbents are used for oxygen and nitrogen generation, but the physical appearance and operating principles of the systems are similar.

Membrane systems use differences in diffusion rates between, for instance, oxygen and nitrogen or hydrogen and CO2 through the walls of specially designed and fabricated hollow polymer tubes.

Information on UIG and UCG products, services and capabilities, can be found on our Products and Services page. A few links to specific types of equipment and services are shown below. The drop-down menu at the top of the page and the "click and go" index on our home page can help you find the pages most relevant to your needs.

If you have a specific need, please use our "Contact Us" button at the bottom of this page to generate an inquiry, or you can phone or fax to allow us to make an initial review of your requirements and preferences; and start the process of defining the best solution for your specific situation.

If your company currently purchases liquid oxygen or nitrogen, or you already have an onsite gas supply system at your facility, and if you believe that changing to a more optimal supply arrangement may require changing your industrial gas supplier, we have tips for you.

Our most important recommendation is to start your search for alternatives as soon as possible. You will need time for to obtain and evaluate well-developed proposals. Your current supplier must receive timely notice of your intent to terminate your existing supply arrangement; and your replacement supplier will need sufficient time to assemble the required plant components and install them. In most cases it is wise to begin two to three years prior to the end of your current supply period.

Useful links to UIG Air Separation Plant Supply and Services Information:

Air Separation Process Technology and Supply System Optimization Overview

Air Separation Process Technology
and Supply System Optimization Overview