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OXIDATION AND DISINFECTION 10.53

Adsorptive Separation. Separation of air into high-purity oxygen and nitrogen gas
streams can be accomplished by preferential adsorption of nitrogen onto a solid adsorbent
as air is passed through a column (or bed). Nitrogen is retained in the adsorbent bed while
oxygen (being less preferentially adsorbed) passes through the column as the product gas
at an oxygen purity between 90% and 95%.
When the column becomes saturated with nitrogen, oxygen production from that col-
umn is discontinued and the adsorbent is regenerated. Regeneration occurs by elevating
the temperature or dropping the pressure in the adsorption column, which reduces the ca-
pacity of the adsorbent. The nitrogen is disturbed (released) from the adsorbent, and the
highly concentrated nitrogen stream is purged from the system. At this point, the column
is returned to the adsorption mode, and production of enriched oxygen is resumed.
Adsorptive separation processes are classified by the method of regeneration. The ad-
sorbent can be regenerated by raising the temperature or by decreasing the pressure. Rais-
ing the temperature of the bed in the regeneration process is known as thermal swing ad-
sorption (TSA). When the pressure is lowered during regeneration, the process is called
either pressure swing adsorption (PSA) or vacuum swing adsorption (VSA). The use of
PSA or VSA technology for air separation is more popular than the TSA process because
of lower capital and operating costs.
The difference between PSA and VSA systems is the operating pressure. A PSA sys-
tem utilizes a compression step of between 30 and 60 psig (209 and 419 kPa) before the
adsorbent beds, and the VSA system operates at between 0.5 and 15 psig (3 and 105 kPa)
for adsorption. Regeneration takes place at near-atmospheric pressure in a PSA system;
the VSA columns are regenerated under vacuum. Thus a pressure change occurs in both
PSA and VSA systems to desorb nitrogen. One characteristic common to both PSA and
VSA is the inability to produce liquid oxygen. This is in contrast to cryogenic air sepa-
ration, which is based on the liquefaction process.
For typical municipal water treatment operations, separation capacity will be used dur-
ing most of the year. Both VSA and PSA oxygen generation have the capability to oper-
ate at low utilization rates. Turndown can be nearly 100%, but with a substantial power
penalty.
Most of the energy losses in an adsorptive system occur during the regeneration of the
adsorbent bed. It is at this time that the energy expended to raise the pressure in a PSA
system is lost (from depressurization of the column) or a pressure drop is induced in a
VSA system (with a vacuum pump). Because the adsorption process utilizes multiple beds
whose operation is set by the timing sequence for adsorption, desorption, and purging,
provisions are made to alter the timing sequence during turndown operation. By extend-
ing the duration of each step, the regeneration step occurs less often and less energy is
lost. This improves the energy efficiency of the process at turndown. These projections
indicate that the VSA system would provide the most energy-efficient operation at ca-
pacity production. The lower energy requirements of the single-train VSA versus the sin-
gle-train PSA continue until approximately 50% of production capacity, at which time the
energy requirements are about equal. Accordingly, the magnitude of the energy savings
offered by the VSA system decreases as the production rate is lowered.
The response time to changes in production rate is almost instantaneous for both types
of adsorptive separation processes. If less oxygen is required, the flow rate through the
system is reduced by adjusting a valve on the outlet of the column. The compressor on a
PSA system would then go into unload mode to maintain the same pressure in the columns,
which lowers the flow rate. For a system utilizing a preset volume of gas through the col-
umn to initiate regeneration, the microprocessor controlling the timing cycle for the beds
would automatically adjust for the lower production rate. This allows quick response to
changes in oxygen production.