Something in the Air

Strictly speaking, non-cryogenic processes are not air separation methods but gas extraction techniques. They provide only one of the component gases of the air - nitrogen or oxygen - leaving behind a mixture of the other of the two gases with the other components of the air, including carbon dioxide, water vapour and argon.

There are two main non-cryogenic processes, both of which rely on adsorption to extract the gases. Pressure-swing adsorption (PSA) pumps pressurised air into vessels containing an adsorption agent - generally zeolite for oxygen generation, carbon molecular sieve for nitrogen. The adsorption agent removes the unwanted gases from the air stream, allowing only the desired gases to pass through. Once the agent is saturated, the pressure in the vessel is reduced, forcing the gas to desorb and regenerating the adsorbant. Generally, PSA plants have two identical vessels, one of which is adsorbing gas while the other is desorbing.

PSA has spawned a variety of variant processes, such as temperature-swing adsorption, where the desorption is triggered by heating the saturated agent, and vacuum-swing adsorption, where the whole process is carried out at reduced pressure.

An alternative, currently only suitable for nitrogen generation, uses a membrane as the adsorbing agent. These use a hollow-fibre membrane, packed into canisters, which allows oxygen, CO2 and water vapour to permeate into its fibres, while nitrogen passes along the fibres as a separate product stream.

Non-cryogenic processes account for only 5 per cent of air separation capacity. However, for installations which supply one customer inside their own premises, they are far more widespread, meeting around 60 per cent of oxygen demand and 25 per cent of nitrogen.

This, explains Jon Ashley, manager of BOC Gases' packaged systems business, is because of the many limitations of non-cryogenic processes. The most important of these is the capacity: `For anything above 50 tonnes per day of nitrogen, non-cryogenic processes can't compete with cryogenic,' he says. Moreover, the processes produce lower quality gases than cryogenic systems. Customers requiring high-purity gases would opt for a cryogenic plant, or buy in the gases in cylinders, says Ashley. Finally, non-cryogenic plants can only supply one gas. If a company needed oxygen and nitrogen, or argon, they would again have to opt for cryogenic or the merchant gas market.

Despite this, use of non-cryogenic plant is growing in some areas. `We're having some success in promoting them as an alternative to buying-in bulk liquid gases,' says Ashley, `particularly for customers who use 10-50 tonnes of oxygen per day, or up to 40 tonnes of nitrogen.'

The technology is developing all the time, says Ashley, with the focus on improving the quality of the gases and making the processes more energy-efficient. On the nitrogen side, membrane technologies are gaining ground for applications which require a steady stream of the gas; they are also highly adjustable, allowing users to select the precise purity of gas required.

In the oxygen area, developers are working on vacuum swing adsorption. This works similarly to pressure-swing adsorption but operates under vacuum, which enhances the efficiency of the molecular sieve and makes the process more energy-efficient. Developments currently in the pipeline include improvements to the selectivity of membranes and molecular sieves, and the scale-up of the processes so that units producing 100tonne/day or more are both feasible and competitive. PE