What is Air Separation?
Air separation is a critical industry that distills and purifies large volumes of atmospheric components from air or manufactured gases and supplies them to other important industries downstream for commercial purposes. Among these are high-purity oxygen, nitrogen, helium, argon as well as rare inert gases. There are different processes available for air separation, and the decision to utilize one over another depends on the target gas, costs involved and, in the case of oxygen, the purity level. Air separation processes can be categorized as cryogenic or non-cryogenic.
What is Cryogenic Air Separation?
Cryogenic distillation is utilized when high-purity product is sought at medium to high production rates. It achieves atmospheric component separation by first lowering the temperature of the air until it liquifies and then selectively isolating each target gas by raising the temperature to its specific boiling point. The capital-intensive operation involves using cryogenic air separation units (ASUs) to produce large amounts of nitrogen and oxygen as well as argon. The ASUs use an air compressor to create the Joule-Thomson effect and lower the air temperature to levels required for liquefaction. The entire setup of distillation columns, heat exchanges and interconnecting pipes must be closely positioned and insulated as well as contained inside ‘cold boxes’ to minimize energy consumption. Once the filtered and compressed air is cooled through the heat exchange and refrigeration process, it is selectively distilled through separation columns at specific boiling temperatures to produce the desired gas products.
Modern designs for ASUs incorporate expansion turbines to increase the efficiency of the compressor.
What is Non-Cryogenic Air Separation?
Non-cryogenic processes are used when producers only require oxygen with up to 90% purity or nitrogen up to 95% oxygen-free. It is also worth mentioning that non-cryogenic air separation will produce the lower purity gases at a reduced rate than a cryogenic process. Non-cryogenic air separation achieves the necessary partition of target gases without the need to liquify air through a refrigeration process. Instead, the process is carried out in ambient temperature using a molecular sieve to separate the desired components of air.
Pressure swing adsorption (PSA) – this process, which is also known as vacuum swing adsorption (VSA) and vacuum-pressure swing adsorption (VPSA), uses zeolite or other absorbents to function as the molecular sieve to initially separate the target gas from air at high-pressures. Different adsorbent material, such as a film with either a specific affinity for oxygen or nitrogen, is then employed to gather the desired end-product.
Membrane technologies – this type of process provides an alternative to separate components from atmospheric air while, at the same time, requiring less energy. The material used to function as the molecular sieve will determine the purity level of the product that can be achieved. For example, in the case of oxygen, polymeric membranes are used for generating 25 to 50% oxygen purity, while purity of 90% or higher will rely on ceramic membranes.
How is the purity of Separated Gases Managed?
Achieving the target purity of separated gas components requires accurately measuring the levels of oxygen during the separation process and when the final product is ready. The presence or absence of oxygen helps operators make precise adjustments with the ‘in-process’ gas. An oxygen analyzer, capable of fast, reliable trace and percent O2 measurements, is necessary to provide immediate feedback during this process or confirm that the final purity level has been achieved.
How does Oxygen get into Non-oxygen Specialty Gas Cylinders?
There are numerous locations where atmospheric oxygen can find its way into specialty gas cylinders. Leaks in cylinder-filling systems is one of the most common sources of unwanted oxygen. Worn-out flexible hoses, industrial type quick-connect fittings and damaged O-rings on CGA connections are all potential sources of oxygen intrusion. Valve packings, worn-out cylinder valves, soft-tip connection points, or loose pipe connections can also allow for oxygen to contaminate the gas. All these items can be resolved with proper maintenance of the filling system and cylinders.
To help make sure you have your system maintenance under control, it is important that you have a trace oxygen analyzer that can accurately detect O2 in single digit ppm levels. If you know the oxygen level in your bulk fill gas, then it is easy to see if you are pulling in any atmospheric oxygen during the filling process. AMI’s trace oxygen analyzers are an excellent choice for monitoring gas quality in gas cylinder filling systems.
How to safely work with inert gases?
The inert gases produced by the Air Separation Industry are stored in cylinders, dewers or large bulk storage tanks. An unexpected leak in any of these containers or the associated process piping and valves can immediately displace the oxygen levels in the surrounding areas and drop it to unsafe levels. Any personnel in the immediate vicinity can become at risk for oxygen deprivation, which can begin as having difficulty breathing and then quickly worsen until possible asphyxiation or death.
The installation and operation of an oxygen deficiency monitor will help safeguard employees while working near inert gas tanks. The monitor will detect when oxygen levels drop below safe levels and sound an alarm to warn personnel to quickly evacuate the area. The oxygen deficiency monitor can also be used to energize relays that turn on ventilation systems to clear an enclosed space of hazardous gas levels.