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

Air separation plants produce the atmospheric industrial gases nitrogen, oxygen and argon using air and electrical power as raw materials. While there are variations in process details, reflecting desired product mix and other factors, all air separation plants employ one of two types of process technology:


	Cryogenic plants - Produce gas (and liquid) products using very low temperature distillation to separate and purify one or more components of air. Cryogenic plants are most commonly used to produce high purity products at medium to high production rates.

	Non-cryogenic plants - Produce gaseous products using near-ambient-temperature separation processes based on differences in properties such as molecular structure, size and mass to generate relatively pure oxygen or nitrogen. Non-cryogenic separation processes are most commonly used when high purify nitrogen or oxygen is not needed (e.g. nitrogen which is 98 to 99.5% oxygen-free, or oxygen at about 93% purity) and when product demand is relatively low; for example: nitrogen at production rates less than about 20,000 scfh / 500 Nm³ or oxygen at production rates less than about 55,000 scfh / 1500 Nm³ .

This page provides an introduction to the characteristics of cryogenic and non-cryogenic plants.

Other Air Separation Technology and Distribution Optimization pages provide more information on the various types of plants, explore inter-relationships between plant type and delivery mode, and discuss other factors which must be addressed to define an optimal gas supply system for a particular application. .

All air separation processes start with compression of air, and additional compressors may be used to boost the pressure of the nitrogen and / or oxygen products leaving the separation and purification process. Even more compression is required to produce large amounts of additional refrigeration when some or all of plant production from an air separation unit (ASU) is desired 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 is used in process heaters, instrumentation systems and cooling systems. Consequently, it is fair to say that electrical power is just as much a raw material as air for the manufacture of 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 tens of megawatts - 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. They 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 (V)	Chemical Symbol
Nitrogen	78.08	75.47	780805	N₂
Oxygen	20.95	23.20	209450	O₂
Argon	0.93	1.28	9340	Ar
Carbon Dioxide	0.038	0.0590	380	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.

Cryogenic argon separation and purification technology is utilized in this plant.

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.

Small cryogenic nitrogen plant relocated by UIG for an industrial gas supplier.

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.

Oxygen and nitrogen gases typically emerge from the air separation process 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.)

When products must be supplied at higher pressures, product compression may be used, or the air separation process technology may be chosen to allow one or more products to be produced at higher pressure (at some sacrifice in air separation / product purification power, but with an overall saving in power and / or capital cost).

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 LOX boil or pumped LOX cycles, which produce products at pressures of 7 barg (100 psig) or more without product compression (but which require additional feed air compression and a special LOX vaporization / feed air cooling heat exchanger system).

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 typically uses nitrogen as the 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 have been commercialized over the past twenty-five years or so. Many relatively small-volume users of oxygen or nitrogen, find that non-cryogenic processes offer a convenient and economical alternative to purchasing gas in high pressure cylinders or buying bulk liquid products to be vaporized to meet demand.

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.

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.

If you have a specific need, please contact us by email, 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 Plant Supply and Services Information:

Air Separation Process Technology and Supply System Optimization Overview

Air Separation Process Technology
and Supply System Optimization Overview