Employing pressure-swing adsorption in an industrial setting
An overall schematic diagram of an in-house PSA generator for creating industrial-scale quantities of nitrogen is shown in
Figure 1. Instrument-quality compressed air (110–140 psig at room temperature) is used as a feedstock that is first filtered,
dried (using a refrigerated air dryer) and then filtered again using a coalescing filter to remove any particulate matter,
oils, and other liquids to protect the carbon bed and valves from contamination and to optimize the process. These filters
are equipped with automatic float drains to empty collected liquids. The purified air is then passed into an air surge tank
(labeled AST101 in Figure 1) to protect the air compressor and dryer from the cyclic air demand of the PSA unit.
Figure 1: Schematic of a typical two-vessel pressure-swing-adsorption system for in-house generation of nitrogen. (FIGURES
COURTESY OF INDUSTRIAL NITROGEN PRODUCTS, FILTRATION AND SEPARATION DIVISION, PARKER HANNIFIN)
The heart of the system is a pair of adsorption vessels that are loaded with carbon molecular sieve (CMS). The CMS generates
nitrogen, which is collected in the nitrogen storage tank (labeled NST101 in Figure 1). Because the adsorption vessels operate
on an alternating basis, one vessel is in the absorption cycle while the other vessel is in the desorption cycle.
The overall control of the generator is performed by a programmable logic controller (PLC) that automatically opens and closes
solenoid valves that pneumatically activate the process valves that control the air and nitrogen flow. At a given time, one
of the two air-inlet valves is open to allow air to flow into a vessel to begin its adsorption cycle, and the exit valve for
that chamber is opened to allow the nitrogen to leave the vessel while the absorption of oxygen is occurring. At the end of
the cycle (which is user settable, typically one minute), valves will open to allow for pressure equalization for 4 s. After
equalization, the air-inlet valve and nitrogen-outlet valve for the other vessel will open and the vessel that had just been
used will be allowed to go into a desorption cycle. Since one of the two adsorption vessels is open (except for the 4 s equalization
time), a continuous flow of nitrogen is produced. After the nitrogen is generated, it passes through a final filter that removes
particulate contamination with an efficiency of 99.9999% at 0.01 μm. An output flow meter and flow control valve are provided
to control the desired flow rate. In many instances, an auxiliary storage tank is employed to provide constant pressure to
the process, even during pressure equalization.
The flow rate of the inlet gas is the primary determinant of the purity of the nitrogen. At a low flow rate, higher-purity
nitrogen will be generated; typically, a PSA system can generate 95–99.999% pure nitrogen depending on the flow rate. Each
system is configured to provide the nitrogen purity required by the application, and an oxygen analyzer is employed to monitor
the oxygen content (the nitrogen is sampled from the nitrogen storage tank).
The nitrogen that is produced by a PSA generator contains approximately 1% argon as this inert gas is not retained by the
CMS (sea-level composition of air is 0.934% argon) and is not a problem for the manufacturing process since it will not react
with compounds of pharmaceutical interest. The nitrogen purity from a PSA generator is frequently defined by the residual
oxygen content (e.g., 1% oxygen in product gas is equivalent to 99% nitrogen and argon).