Industrial Oxygen Generation

The separation of high-purity industrial gases is a specific process that is based on critical separation stages where a highly specialized molecular sieve is essential. The process uses the molecular sieve to produce high-purity oxygen from an air stream by adsorption of the basic components of the air.

Oxygen generation is necessary for glass production, steel manufacturing and mining as well as wastewater treatment facilities where it is necessary to have consistent, dependable, secure supplies of high-purity oxygen.

Industrial oxygen generation can apply to one of several areas:

Cryogenic separation is the term used for processes that involve air separation units (ASUs). In cryogenic air separation, the air is divided into its basic primary components of oxygen, nitrogen and other trace gases, including argon. This is accomplished by passing the air through the molecular sieve to remove carbon dioxide, water, nitrous oxide and small hydrocarbons and then passing that ultra-dry air stream into a cold box (i.e., at cryogenic temperatures) where the remaining components fractionate into ultra-pure forms of oxygen, nitrogen, argon and noble gases. It is essential that the molecular sieve is highly dynamic for impurity removal, possesses excellent adsorption and desorption kinetics and has excellent physical properties. Cryogenic separation is a method used to generate oxygen with a purity of greater than 99.9%.

Pressure swing adsorption (PSA) separation is another area of industrial oxygen generation. PSA technology takes advantage of the affinity of the components in the air for the molecular sieve under pressure. In PSA oxygen generation, the pressure is increased to a point where the nitrogen affinity for the molecular sieve is far greater than that of oxygen. The molecular sieve adsorbs the nitrogen, and an oxygen rich stream – up to approximately 95% purity – of air remains. The pressure is then decreased to release the nitrogen and effectively regenerate the bed so the process can be repeated.

Vacuum pressure swing adsorption (VPSA) takes advantage of the same technology as PSA; however, VPSA uses vacuum pressure, or the difference between atmospheric pressure and absolute pressure generated by a vacuum blower. The vacuum swing allows for maximization of the adsorption and desorption by taking advantage of the full isotherm curve for the lithium molecular sieve. The VPSA process is applied in production plants, skid-mounted units and medical oxygen units. It has the benefit of providing high-purity oxygen with a compact footprint and power savings compared to ASU and PSA.

Applications

Cryogenic Oxygen Generation

Cryogenic Oxygen Generation is a technology pioneered by Dr. Carl von Linde in the early 20th century, and it is still used today to produce large volumes of oxygen and nitrogen at high purities.

The cryogenic process consists of compressing, cooling and liquefying atmospheric air and then separating its components by distillation.

Airborne impurities in the inlet air, such as water and carbon dioxide, must be removed before cooling and liquefying the air to keep it from freezing and solidifying in the cryogenic equipment, thus reducing its efficiency or even plugging it.

Air purification from water, carbon dioxide and other impurities, such as hydrocarbons and nitrogen oxide, is achieved by using adsorbents.

Typically, water is removed by activated alumina, while carbon dioxide and other impurities are effectively captured by using 13X type molecular sieves.

The air purification section of a cryogenic oxygen plant is, in the majority of the cases, based on the temperature swing adsorption (TSA) technology; normally there are two adsorbing beds, with one bed in adsorption and one in regeneration at any given time. When the bed in adsorption is saturated with water and carbon dioxide, it switches to regeneration, while the fresh bed having just gone through regeneration goes online. Regeneration is done by heating the bed to a certain temperature (typically 140–180°C) by flowing a dry and carbon dioxide-free regeneration gas through it. Impurities desorb from the adsorbent at those temperatures and are carried out from the bed by the regeneration gas.

Zeochem has extensive expertise in this adsorbent application, gained through many years working in close cooperation with technology providers and plant users.

Our portfolio includes activated alumina and various 13X type sieves with characteristics and performances tailored for each specific air purification application.

Furthermore, we have the capability to perform design simulations for air purification duties based on TSA technology; this capability perfectly complements our product offerings, allowing us to propose an optimized solution to meet clients’ needs and demands.

Oxygen PSA

Oxygen PSA is a method for manufacturing industrial and medical oxygen based on pressure swing adsorption (PSA) technology.

The technology consists of direct oxygen concentration by adsorbing the nitrogen in the air onto a molecular sieve.

The oxygen PSA units are generally made up of two molecular sieve beds, with one bed in adsorption at high pressure and one in regeneration at low pressure at any given time.

The air is typically compressed at 4–6 bar and then chilled to a low temperature to decrease its dew point by removing some water through condensation. The air then finally flows through the molecular sieve bed in adsorption. Nitrogen is adsorbed onto the sieve, and concentrated oxygen flows out of the bed and is collected in a buffer tank. Due to the high concentration of nitrogen in the air, the molecular sieve bed is quickly saturated with nitrogen, requiring the switch to regeneration at short intervals (typically 60 seconds or less); molecular sieve regeneration is achieved by releasing the pressure and purging with a fraction of the outlet concentrated oxygen.

Due to thermodynamic limitations and low adsorption capacity of standard sieve types for argon, this technology concentrates oxygen up to a maximum of ca. 95%; nevertheless, the achievable outlet oxygen purity depends on the type and performance of the installed sieve as well as on the unit’s design and operation.

Due to the power needed to compress the air (and associated costs), this technology is mainly applied in small-size/skid-mounted type PSA units.

These units are generally in service only in applications where a small amount of oxygen is needed and it is either logistically complex or simply too expensive to provide it via traditional ways (e.g., pipeline, bottled oxygen or liquid oxygen by truck).

Examples include industrial applications (e.g., aquaculture installations, ozone generation for wastewater treatment, small mining and smelting facilities, etc.) as well as medical applications (centralized oxygen supply of a hospital or veterinary clinic, room oxygen enrichment, military and remote field hospitals, etc.).

The molecular sieve type used in those units has been a 5A, but now manufacturers of oxygen PSA units are switching to higher-performing 13X type sieves. Also, LiLSX type sieves have been successfully applied in oxygen PSA.

We have an extensive portfolio of molecular sieves for oxygen PSA – including 5A, 13X and LiLSX types – and can assist in selecting the proper material for each specific application and unit design.

Vacuum PSA

Vacuum pressure swing adsorption (VPSA) is a process that operates above atmospheric pressure and capitalizes on the affinity of the components in the air toward the molecular sieve. The molecular sieve for VPSA is traditionally a lithium-based form of the aluminosilicate zeolite. The molecular sieve directly adsorbs the nitrogen in the air inlet to produce high-purity oxygen streams of gas.

VPSA plants or units traditionally consist of four components: a feed blower, molecular sieve beds, a vacuum blower and a product or surge tank.

The process has two main steps: an adsorption phase and a regeneration phase. The regeneration phase consists of depressurization with a vacuum to below atmospheric pressure, an oxygen purge and repressurization of the vessel. VPSA systems traditionally exhibit two sieve beds. In operation, the feed blower supplies a constant stream of air to the system. The air is passed through one molecular sieve bed where nitrogen, water, carbon dioxide and trace hydrocarbons are adsorbed preferentially by the molecular sieve, leaving a high-purity stream of oxygen gas. This high-purity stream of oxygen is collected in the surge tank. It is important to note that the molecular sieve beds operate in sync; while the first bed is experiencing adsorption, the second bed, which is off stream, is undergoing a regeneration process under vacuum. In this process, the nitrogen, water, carbon dioxide and trace hydrocarbons are desorbed. Once desorption is completed, the bed is further regenerated with a small amount of the high-purity oxygen from the process to remove any remaining contamination from the adsorption process and to repressurize the bed for the next adsorption.

VPSA plants or units are crucial for performance in the steel-making process, glass production, mining and wastewater treatment facilities where it is necessary to have consistent, dependable, secure supplies of high-purity oxygen. It is necessary to not only have a reliable process, but to also have a facility that occupies a small footprint. VPSA technology, through the utilization of advanced molecular sieves, has evolved to meet the needs for high-purity oxygen by a plant or unit with a compact footprint.

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