What is Pressure Swing Adsorption (PSA)
Pressure Swing Adsorption or PSA is an economic and reliable method used to separate mixed gas into individual gases while achieving a high purity level. PSA is a non-cryogenic air separation process which essentially means it is a process which uses near ambient temperatures for the production of nitrogen or oxygen in contrast to the cryogenic distillation techniques of gas separation which take place are very low temperatures and is a process commonly employed in chemical and petrochemical processes in commercial practices.
During the process, gases are separated under pressure based on the species molecular characteristics and affinity for an adsorbent material. PSA is used to recover hydrogen from coking or conversion gases or to split oxygen and nitrogen from the air.
The Principle of Pressure Swing Adsorption
In pressure swing adsorption, specialised adsorbent materials adsorb the gas molecules such as oxygen, carbon dioxide, water vapour and other gases under high pressure with the exception of nitrogen. PSA systems for oxygen production were first used during the 1970’s and since then the technology has overall shown rapid improvement.
Before getting into the process of how the PSA System works it is essential to understand the difference between adsorption and absorption. The Pressure Swing Adsorption separation system is based on the principle of adsorption. Adsorption is a surface-based procedure while absorption involves the whole volume of the material. The adsorption method is when the gas or liquid molecules adhere on the surface of the adsorbent. This process creates a film of the adsorbate on the surface of the adsorbent.
Each PSA system uses specialised adsorbent materials such as zeolites, molecular sieves, activated carbon etc. These substances are used as a trap, ideally adsorbing the target gas species at high pressure. The process then swings to low pressure in order to desorb the adsorbed material hence the name pressure swing adsorption. This adsorption process is based upon the gas molecules binding to these absorbent materials, preferably only the gas, which is to be absorbed, while all other gases in the mixture pass through the adsorbent bed.
The Process
A PSA System essentially consists of two or more adoption towers filled with absorbent materials, filters, and a storage tank. One of the benefits of a PSA system is that you can stabilise the nitrogen purity of the gas. This is done by the manufacture of the generator based on 2 main points: The number of absorbent materials in the tower and amount of time the air is in the towers. Of course, the quality of the materials used will also have an effect on the purity of the gas produced.
Apex’s PSA generators use Carbon Molecular Sieves (CMS). The quantity of the CMS required for the generators is based on a number of factors such as the purity required, contact time, cycle time, velocity, and temperature. CMS is generally known to be a strong and chemically inert material and are cylindrical in shape, this is intentional in order to obtain a high surface to mass ratio. Two types of filters are commonly used in Nitrogen PSA systems, pre-filters, and after-filters.
Pre-filters are a coalescing type of filter used to separate fluid mixtures into individual using the principal coalescence. Coalescence is a process where fluid molecules come together to form a larger whole and separate particulate component of mixtures at comparable efficiencies as particulate filters. Essentially, they are installed in nitrogen generator inlets in order to avoid liquid from flowing into the adsorption towers. If liquid comes into contact with the CMS material, then its performance is significantly affected.
After-filters are installed at the outlet of the generators in order to protect any downstream equipment from desiccant dust. The after-filter element is designed to remove any solid particulates from the compressed air. These hybrid pleated filters provide high dirt retention, low-pressure drop, and long element life.
How It Works
Gas enters the inlets and is passes through pre-filters and removes any water from the gas, the compressed purified air then passes through onto the adsorption towers. Predominantly oxygen is adsorbed on the carbon molecular sieve (CMS) and nitrogen enriched gas passes on from the tower into the storage tank of the generator.
The concentration of the oxygen can be reduced to meet required purity levels. During adsorption in one tower the second tower is totally regenerated just by depressurisation to ambient pressure. Regeneration can also be referred to as “purging” and is the process in which the gases with the exception of nitrogen is accumulated during the cycle being stripped away. The Oxygen enriched off gas is then vented to the outside atmosphere and after so many minutes adsorption in one tower switches over to the second tower and the first one is regenerated.
[1] Pressurisation of the Inlet Gas [2] Adsorption of the inlet gas at a high pressure [3] depressurisation to the atmospheric pressure, where it releases C02, at the bottom of the desorption column and [4] desorption of C02 gas from the adsorbent with purging gas. The gases are then introduced into the first tower and pressurised, resulting in the C02 adsorption. The applied pressure is then transferred to the second tower. While the second tower is pressurised the first tower is depressurised, and the carbon dioxide is separated. During the desorption steps, the inlet C02 gas stream is stopped and N2 is only introduced to desorb C02 after the depressurisation. This cycle then continues switching from an adsorption tower to a desorption tower.
So essentially, Pressure Swing Adsorption (PSA) is the process applied to separate gas mixtures such as carbon dioxide from other gases, and is used for situations such as ammonia production and hydrogen purification etc. In terms of the high-cost effectiveness, PSA is commonly viewed as an extremely appealing approach due to its simple operation, high performance at ambient temperatures, high regeneration rate and low energy intensity.