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Nitrogen Membrane Systems

Membrane nitrogen generators use tube bundles made of special polymers, often configured in a manner similar to a shell and tube heat exchanger.  The air separation principle is that different gases have different permeation rates through the polymer film.  Oxygen (plus water vapor and carbon dioxide) are considered "fast gases" that diffuse more rapidly through the tube walls than the "slow gases" argon and nitrogen.  This allows dry air to be converted to a product that is an inert mix of mostly nitrogen gas and argon, and a low-pressure "permeate" or waste gas that is enriched in oxygen (plus water vapor and carbon dioxide) and vented from the shell.

Nitrogen product emerges from membrane units at close to the compressed air feed pressure.  In many applications this means that no supplemental product compression is required. Because there are no moving parts in the separation process, membrane units can be rapidly activated when needed and shut down when they are not. 

Membrane separation units are typically made in standard-size modules, with nitrogen production ratings that depend upon the desired nitrogen purity.  For a given standard module, nitrogen production rates increase with higher inlet air flow rates, but the purity of the product decreases. When required production capacity (at a specified purity level) exceeds the largest standard module size, a number of smaller units will be manifolded to allow them to operate in parallel. 

Membrane units are most cost effective for relatively low demand applications.  Because larger-capacity units are typically made up of multiple smaller capacity modules, membrane units have a close-to-constant cost per unit of separation/ production capacity over a wide range of production rates, in contrast to the declining cost for marginal capacity that is typical with PSA nitrogen generators and cryogenic air separation/ nitrogen generator systems.

There is a trend, however, toward using membranes at higher production rates.  This has been made possible by manufacturers increasing the physical size and production capacity of their largest modules.


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