Efficient working of air separation plants is crucial for smooth operations of all kinds of manufacturing and engineering industries. It is imperative for businesses to get access to oxygen and nitrogen with specified volume and purity.
Mostly, we use two methods of separating oxygen and nitrogen from ambient air—cryogenic and non-cryogenic. Both are excellent techniques for onsite generation of the gases for industrial and medical applications. Cryogenic air separation process takes atmospheric air which is liquefied and then distilled based on the boiling points of oxygen and nitrogen. And the non-cryogenic processes including pressure swing adsorption (PSA), vacuum pressure swing adsorption (VPSA) and membrane separation are used for separating oxygen and nitrogen through adsorption.
Non-cryogenic techniques of air separation are considered ideal for lower volume generation where the primary focus is on the cost-effectiveness with acceptability of even lower purity levels. Oxygen and nitrogen can be produced with purity between 85%-95% using PSA processes. However, purity greater than 95% is also achievable with non-cryogenic techniques.
On the other, cryogenic air separation is best suited for high volume production of oxygen and nitrogen. A cryogenic oxygen/nitrogen plant can generate oxygen with purity up to 99.7% and nitrogen with purity up to 99.99%. It is considered as the most efficient air separation process fabricated with the latest cryogenic technology.
Difference between cryogenic and non-cryogenic process
Firstly, both cryogenic and non-cryogenic air separation processes require compression of the atmospheric air leading to removal of the heat of the compression. Next cooling of the process air is needed which is provides either by exchangers or through aftercoolers with cooling water and chilled water.
The two processes are completely different in the downstream of cooling air. For an efficient operation of cryogenic oxygen/nitrogen plant it is extremely important that impurities such as moisture, carbon dioxide and hydrocarbons are removed from the air, which is accomplished by temperature swing adsorption (TSA). The adsorber vessels are made up of activated alumina and molecular sieves. The vessels work sequentially between adsorption and regeneration.
In the non-cryogenic process based PSA or VPSA oxygen/nitrogen plants impurities like hydrocarbons, carbon dioxide and moisture are eliminated with the help of carbon towers. After purification, the process air enters a pressure swing adsorption tin-bed which comprises of a molecular sieve. Oxygen and carbon dioxide are separated from the air in the carbon molecular sieve.
However, it is to be noted that different adsorbents attract certain gases more strongly comparison with other gases. Citing the example of nitrogen production, it is observed that oxygen is very strongly adsorbed on the bed while nitrogen-rich stream is recovered as the desired product. Exhaust gases are pushed back into the atmosphere. Membrane based air separation technology can generate oxygen/nitrogen with purity between 95-99.5% with volumes up to 30t/d whereas PSA/VPSA can generate volume typically between 5-500t/d.
Pick the right air separation technology
Both cryogenic and non-cryogenic air separation technologies have their strengths and advantages based on volume, purity and cost. Knowing your requirements and using the most suitable process for meeting the requirements is the best approach for successfully running your business.
Cryogenic process is considered as the ideal choice if your requirements focus on high purity large scale production unencumbered by budget. No denying the process is capital-intensive but it can generate uninterrupted supply of high purity oxygen/nitrogen. Most of the large and mid scale businesses prefer using the cryogenic method.
On the other hand, non-cryogenic processes such PSA or VPSA are also excellent for meeting oxygen/nitrogen requirements of enterprises driven by cost-effectiveness and not much focused on efficiency and purity.