| PSA Gas Purification Process |  |
 |
Typically Gas purification systems can be Cryogenic separations or Non-cryogenic separations. Cryogenic methods of air separation are the traditional process of producing nitrogen and oxygen gases. But this technology requires a huge set up as large plants are needed to cool air to below zero by several hundred degrees for separating the component gases. Usually gases are then distributed to customers in liquid form using tankers Compared to this the Non-cryogenic air separation process is a more recent technology. Non-cryogenic techniques were first developed during the 1970s on-site applications.
It involves forcing of air through special materials that has the capacity to selectively pass or retain particular gases. These new systems offer much leverage in terms of space than cryogenic plants and can be placed directly on the customer's site. Non-cryogenic gas separation processes such as Pressure Swing adsorption technology and membrane based separations have completely altered the economics of certain processes and is playing a major role in expanding the markets, applications and uses of gases. However a number of factors need to be considered before opting for a particular process.
Selection of the Purification Process:
When it comes to the purification process there are several factors that influences the choice of separation technology. However the following criterions must be followed for selecting the process of separation:
- Volume requirement: Cryogenic separation is economical for large tonnage users.
- Applications requiring low temperatures: Only cryogenic systems can give the liquefied gases that are must for low temperature applications such as freezing of food.
- Purity requirement: Non-cryogenic systems are generally found to be wanting when it comes to achieving high purities economically, but comparatively less pure products may be sufficient for some applications.
- Continuity of supply: Cases where demand is fluctuating then it is resonable enough to opt for liquid storage tanks that gets filled by tankers or an on-site plant. Another point to be noted is that if a gas supply is a very important necessity, then perhaps for safety reasons, a non-cryogenic system needs to be backed up by a liquid storage in case of emergency.
- Customer location: Some places can be too remote for economical delivery of liquid supplies by road tanker or may be out of reach totally. In those cases Non-cryogenic sources is the only viable option.
- Type of impurities to be removed.
Non-cryogenic purification techniques may include Membrane separation, Pressure Swing adsorption(PSA) and a variation of PSA called Thermal Swing Adsorption.
- Thermal Swing Adsorption: As we would be dealing
extensively in PSA a comparison is given between PSA and TSA. The
graph shows the adsorbed impurity loading being a function of
pressure and temperature. In a typical Temperature-swing adsorption
(TSA) cycle, the regeneration process gets carried out at constant
pressures. This is done by reduction in the loading by increasing
the temperature. Whereas in a PSA cycle, there is an option that the
loading can be reduced at constant temperature by lowering the
pressure.
- Vacuum Swing Adsorption: In this process of Vacuum swing
Adsorption (VSA), compressed air is put into a tank that are filled
with molecular sieves. The sieves contain absorbents that gathers
the unwanted gasses. Once the sieves get saturated with the unwanted
gasses, a vacuum pump is turned on and it sucks the sieves clear of
the undesired gases. This allows the sieve to be reused. Two tanks
are used in this process, to make the product available
continuously. The process is almost similar to Pressure Swing
Adsorption process except that in VSA, differential pressures
generally happen at lower absolute pressures.
;)
| Gas | Gas
Volume Nm3/hour |
Purity % by volume |
Plant type | Notes |
| H2 | Up to about 400 | > 99. 9 | Trailer up to about 400 Kg H2 per hour | Gaseous |
| 100 to 1,00,000 | > 99. 9 | PSA, on-site supply | Recovery from residual refinery gases. Load range typically 50 to 100% |
|
| 300 to 2,00,000 | >99. 9 | Steam reformer plus CO shift plus PSA, On-site supply | Load range typically 50 to100% | |
| N2 | 0 to about 1,000 | > 99. 99% | Liquid from tank | Also for highly variable withdrawals |
| About 50 to 1,000 | <99% | N2 membrane, On-site supply | Load range typically 30-100% | |
| About 100 to 5,000 | <99. 5% | N2 PSA, On-site supply | Load range typically 30-100% | |
| About 200 to 3,00,000 | >99. 99% | Cryogenic air separator, On-site supply | Load range typically 50-100% | |
| O2 | 0 to about 1,000 | >99. 5% | Liquid from tank | Also for highly variable withdrawals |
| About 300 to 5,000 | <94% | O2 PSA (VPSA), On-site supply | Load range typically 30-100% | |
| About 1,000 to 1,00,000 | >99. 5% | Cryogenic air separator, On-site supply | Load range typically 50-100% |
Types of PSA Gas Purification Process: