The importance of an in-house pre purification unit in a Cryogenic Process
The pre-purification unit (PPU) plays a key role in the cryogenic process of producing liquid and gaseous nitrogen and oxygen.
Let’s take a step back in understanding how a PPU works, and what are the critical aspects to be considered. This unit is designed to remove condensate and carbon dioxide (CO2) from compressed air, before entering the primary heat exchanger of the cold-box. This critical treatment is required to avoid the clogging of the exchanger itself due to the freezing process.
PPU is usually composed of one stage of moisture separation and dedicated vessels containing alumina and molecular sieves. The variable combination of those absorbent materials provides a high degree of removal of the critical molecules. The process inside the TSA (Temperature Swing Adsorption) part of PPU, can be divided into such phases:
- Adsorption: Compressed air containing moisture and CO2 goes through the vessel containing alumina and molecular sieves. The alumina adsorbs moisture, while the molecular sieves capture CO2. Over time, the capacity of these adsorbents becomes saturated as they accumulate contaminants.
- Regeneration: Temperature Swing Adsorption (TSA) is an effective method for regenerating vessels filled with adsorbents like alumina and molecular sieves that have been used for moisture and CO2 removal.
- Heating the Vessel: The vessel containing the saturated adsorbents is heated using controlled temperature increase, usually up to 140°C to 180°C, a level suitable for desorbing the accumulated moisture and CO2 from the adsorbents.
- Desorption of Contaminants: As the temperature rises, the moisture and CO2 that have been adsorbed by the alumina and molecular sieves are released. The elevated temperature reduces the adsorption capacity of the adsorbents for these contaminants, allowing them to be expelled from the adsorbent materials.
- Purge Gas: To enhance the removal of desorbed contaminants, a purge gas is injected into the vessel. This gas, waste nitrogen coming from the cryogenic process as a by-product, helps carry away the desorbed moisture and CO2 from the vessel and out through a dedicated exhaust or venting system in a safe manner.
- Cooling Down: After the contaminants have been successfully removed, the vessel is gradually cooled back to its normal operating temperature.
- Reactivation: Once the vessel has cooled down, the adsorbents—now free of moisture and CO2—are reactivated and ready to resume their adsorption duties. The vessel is then put back into service for the adsorption phase, where it will again remove moisture and CO2 from the incoming compressed air.
- Monitoring and Control: Throughout the TSA process, temperature, pressure, and other relevant parameters are monitored and controlled to ensure effective regeneration and to prevent damage to the adsorbents or equipment.
- Final Filtration: The air may then be passed through additional filters to remove any remaining particulates or contaminants.
- Dry and Clean Air Output: The purified, dry, and CO2-free air exits the unit, ready for use in various applications and processes, where very high-quality compressed air is required, like the cryogenic one.
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Sinergia PPUs guarantee:
✅ Full Control of air quality incoming the Separation Column:
By managing the in-house design and construction , Sinergia ensures precise adjustments and optimized operational conditions of the PPU.
✅ Leverage of Extensive Experience in Air Treatment:
Thanks to years of expertise in gas treatment, Sinergia has improved the selective criteria of its suppliers, improving the efficiency of its systems.
✅ Enhanced Understanding of the Performance at the Separation Column Outlet:
By controlling the input, Sinergia gains a deeper comprehension of the entire separation process. This allows the company to fine-tune the performance, ensuring superior results at the outlet of the separation column.
