In this article, we will learn about the Pressure Swing Adsorption Working Principle for Nitrogen Generation.
The Pressure Swing Adsorption(PSA) separation system based on the principle of adsorption. Adsorption process, in which 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.
Don’t confuse both adsorption and absorption. Adsorption is a surface-based process while absorption involves the whole volume of the material.
The PSA system is convenient for on-site production lower than 2000 Nm3/h of nitrogen with purities ranging from 95 to 99.9%.
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Principle Of Pressure Swing Adsorption
In the pressure swing adsorption, Carbon Molecular Sieves (CMS) used as an adsorption medium to adsorb the gas molecules such as Oxygen, Carbon dioxide, Water vapour and Other gases (except Nitrogen) under high pressure. Because these gases adsorbed on the surface of the carbon molecular sieves at a faster rate than the nitrogen molecules.
Working Of Pressure Swing Adsorption
Normally, the instrument air from the compressor feeds to the Pressure Swing Adsorption(PSA) system. The PSA system consists of two separate but identical pressure vessels commonly referred to as “dual” or “twin” towers. In that one tower undergoes Adsorption cycle (Online) and another tower in the regeneration cycle (off Line). As a minimum, the adsorbing towers shall be sized for 120% of the design capacity of Nitrogen generation. Due to this configuration, automatic (and cyclic) switching from generation to regenerated and vice-versa.
The towers are filled with activated Carbon Molecular Sieves(CMS). The Carbon molecular sieves are in a cylindrical shape, to obtain high surface to mass ratio. Carbon Molecular Sieves physically tough and chemically inert material.
When the compressed air flows up through the “online” tower, except nitrogen all other gas molecules adhere on the surface of CMS. Then the Nitrogen discharged from the chamber into the distribution system.
Regeneration also referred to as purging. It’s the process in which the gases (except Nitrogen) accumulated during the “online” cycle stripped away during the “offline” cycle. Low-pressure dry purge Nitrogen flows gently through the regenerating bed (Refer to the above figure, it’s shown in Orange colour line). The tower to be regenerated quickly and depressurized to the atmosphere. Due to fast depressurization allows the oxygen desorption (the oxygen vented at the safe location; particular attention shall be given to the vented gas because it’s enriched with oxygen).
Carbon Molecular Sieves (CMS)
The quantity of the Carbon Molecular Sieves (CMS) required for Nitrogen generation is based on the following factors
- The purity of Nitrogen required,
- Contact time,
- Cycle time,
- Velocity,
- Temperature
Usually the compressed air flow in the twin tower with a velocity of about 4.5 meters per minute to prevent CMS dusting and bed fluidization. The CMS can use for years without changing it. The minimum life of CMS around 5 years. Once the nitrogen purity will start to degrade, it’s an indication for replacement. Normally an oxygen analyzer indicates performance and controls nitrogen purity.
Filters
Two types of filters used in the Nitrogen generation system. They are
- Pre-Filter
- After-Filter
Pre-Filter
It’s a coalescing type filter, installed at nitrogen generator inlet to prevent the liquid particle enter into the adsorption tower. If liquid particle contact with CMS material, then the performance of CMS reduced.
Normally the pre-filter has sized to remove liquids and reducing the contamination level of the compressed air to 0.1 or 0.01 PPM. An integrated local differential pressure indicator used to determine element condition and automatic drain valve is provided on systems.
After-Filter
After-filter installed at the nitrogen generator outlet to protect downstream equipment from desiccant dust. The after-filter element designed to remove solid particulates from the compressed air. The hybrid pleated filter media provides high dirt retention, low-pressure drop, and long element life. The element removing particles 1 micron and larger, again integrating a local indicator to determine element condition. The manual valve is ever before and after filter to insulate the element during cartridge changing.
Points to Ensure while Selecting Pressure Swing Adsorption System
- The instrument air temperature range generally suitable for molecular sieves operation (generally, the temperature shall not be higher than 60°C),
- Normally, an inlet filter included in the molecular sieve (PSA) type generator package. In order to reduce the molecular sieves plugging frequency. The compressed air quality requirements shall be 0.01 micron as filtration grade and 0.003 ppm as max oil quantity.
- The instrument air pressure and flow rate available at molecular sieve (PSA) type generator battery limit shall be always checked with the generator supplier requirements.
- As a thumb rule, the adsorption tower (twin tower) sized for 120% of the required capacity. In order to taken care of Nitrogen losses and the future requirement.
- After filters used to remove the particle size of 1 micron with filter rate efficiency of about 99.9%.
Conclusion
The PSA systems suitable for producing nitrogen up to the capacity of 2000 Nm3/h with purity ranging from 95 to 99.9%. The capital and maintenance cost of the PSA system is very less when compared with other nitrogen generation system.