Page 216 - Adsorption Technology & Design, Elsevier (1998)

Selected adsorption processes 199

nitrogen is adsorbed to a much greater extent than oxygen and this is clearly
shown in Figure 7.5. Hence, during the basic two-bed Skarstrom cycle for
producing oxygen of up to 95% purity on a small scale (e.g. for medical use)
oxygen is recovered during the second step of the process when the two beds
are connected together following pressurization of the first column with air.
Nitrogen is retained in the bed while oxygen (being the least strongly
adsorbed component) passes through bed 1 and is collected concomitantly
as it flows through bed 2, thus purging the latter bed by countercurrent flow
(known as back purging). The oxygen recovery is, however, less than 25%
and would be quite uneconomic for the large-scale production of oxygen.
Units producing large throughputs of oxygen utilize six steps comprising
adsorption, pressure equalization, desorption, back purging, a pressure
equalization step and repressurization; this arrangement gives better oxygen
recovery. These more complex processes have been reviewed by Davis
(1972). A process involving vacuum desorption of nitrogen (and other
contaminants such as carbon dioxide and water vapour) has proved to be of
commercial value (Sircar and Zondlo 1977). Such a system is capable of
producing oxygen of 93 % purity (balance N2 and Ar) at throughputs of 100
tonnes per day. Such plants are now in wide use. Vacuum swing adsorption
has two major advantages compared with a pressure swing adsorption
operation of similar capacity. First the adsorbent capacity is higher under
vacuum operation than pressure swing operation, thus allowing more
nitrogen to be adsorbed during the adsorption step, and secondly, there is a
smaller demand on energy use with savings of up to 30%. Energy savings
accrue because the feed does not require as much compression- just
sufficient to overcome the pressure drop of the adsorbent beds. Only three
basic steps are necessary for the vacuum adsorption process described and
which is illustrated for a three-bed system in Figure 7.6. The steps are
adsorption, desorption and repressurization. During the adsorption step air
is fed to one of the adsorbent beds by a low pressure blower. Water vapour,
carbon dioxide and nitrogen are selectively adsorbed in the bed thus
allowing high purity oxygen to pass through the bed to be delivered as
product. Oxygen purity declines slightly during the complete cycle of events.
The time interval for the adsorption step is thus set to give a specific average
oxygen purity. Nitrogen, water vapour and carbon dioxide contaminants are
removed during the desorption interval by applying vacuum to the
adsorbent bed. The remaining step in the whole cycle is repressurization of
the bed which occurs by using atmospheric air together with a fraction of the
product oxygen stream. Cycle times for the process are of the order of two
minutes.
The production of nitrogen, as opposed to oxygen, is achieved using a
molecular sieve carbon which preferentially adsorbs oxygen. Figure 2.5