Page 219 - Adsorption Technology & Design, Elsevier (1998)

Selected adsorption processes 201

illustrates the much faster rate of uptake of 02 by a molecular sieve carbon
than nitrogen. The rate of adsorption of O2 is faster than that of N2 by a
factor of approximately 2.5 x 102 and is due to the large diffusion coefficient
of O2 into particles of molecular sieve carbon in comparison with N2. During
the adsorption step of a PSA process, therefore, oxygen is preferentially
retained in the adsorbent bed and nitrogen passes through and may be
collected. The Bergbau-Forschung process is a simple process for producing
N2 from air and involves a two-step cycle (Knoblauch 1978). During the first
step of about one minute interval, adsorption of oxygen occurs at about 3 to
5 bar pressure. The second step is countercurrent evacuation at approxim-
ately 0.1 bar pressure and is also of about one minute duration. It is reported
that at an adsorbing pressure of 5 bar, the flow of product N2 at 98% purity is
approximately 40 m3h -1. The balance of gas in the N2 product is argon.
Nitrogen recovery from air by this process is about 50%. The desorption
product obtained in the countercurrent evacuation step contains about
35% 02 (the balance being N2, CO2 and water vapour). If an intermediate
purge step is introduced into the cycle, the purity of the desorbed oxygen
product can be considerably enhanced and utilized for other process
operations.

7.4 THERMAL SWING ADSORPTION (TSA) PROCESSES

7.4.1 Two-bed systems
Four operating steps comprise the basic two-bed thermal swing process (see
Figure 5.18). Separation of components occurs during the first (adsorption)
step of the thermal swing cycle, the most strongly adsorbed component
being retained in the adsorbent bed while the least strongly adsorbed
component passes through the bed. Thus the feed mixture containing an
adsorbate at a partial pressure p~ is passed through the adsorbent bed,
operating at a temperature T1, where the adsorbate is wholly or partially
removed from the feed stream. The loading of adsorbate on the adsorbent at
this first stage is q~ (see Figure 5.14b). When regeneration of bed 1 is
required its temperature is raised to T2 by passing hot feed, hot inert purge
or steam through the bed. Raising the temperature of bed 1 to T2 constitutes
the second stage of the thermal swing process during which the adsorbate
loading diminishes to q2. The third stage of the process cycle is that of bed
regeneration when bed 1 is heated to an elevated temperature with either
hot feed or a hot purge gas. The final process stage is when bed I is cooled to
the original temperature T1 (step 4).