Page 46 - Adsorbents fundamentals and applications

TEMPERATURE SWING ADSORPTION AND PRESSURE SWING ADSORPTION 31

concentration and flow rate from a PSA process, without the use of surge or
mixing tanks, vary with time. Product recovery measures the amount of com-
ponent that is contained in the product stream divided by the amount of the
same component in the feed mixture. Adsorbent productivity is measured by the
amount of product or feed mixture processed per unit amount of sorbent per
unit time. For a given separation, the product purity is predetermined, the energy
requirement is usually proportional to the recovery, and the size of the sorbent
bed is inversely proportional to the sorbent productivity. It is important to bear in
mind that these three parameters are interrelated for any given PSA process, and
the interrelationship can be determined only through a model for the process. In
addition to these parameters, the term “strong adsorptive” will be used to refer
to the strongly adsorbable gas, whereas “weak adsorptive” will be used for the
weakly adsorbable gas. The product that mostly contains the weak adsorptive
may also be called “loosely light product” or “raffinate”, the latter is in analogy
to liquid–liquid extraction.
The basic PSA cycle was described by Skarstrom in 1960 (Skarstrom, 1960;
1972). A similar cycle was the Guerin–Domine Cycle, invented at about the
same time (Guerin–Domine, 1964; see a detailed account of these inventions
in Yang, 1987). The latter was the basis for the modern vacuum swing cycle.
The major additions to these cycles are co-current depressurization (CD) and
pressure equalization (PE) steps. The CD step was added to increase product
recovery, whereas the pressure equalization step was added to conserve the
mechanical energy.

Skarstrom Cycle. The Skarstrom cycle uses a two-bed apparatus as shown in
Figure 3.1. There are two main cycle steps: high-pressure adsorption and low-
pressure purge. In between these two steps are two pressure-changing steps. These
cycle steps are illustrated in Figure 3.2. After the high-pressure adsorption step,
the first bed is depressurized to atmospheric (or a low) pressure. At the same
time the compressed feed mixture is switched to the second bed to repressurize
it. It then starts the adsorption step at the feed pressure. A fraction of the purified
effluent from the second bed is passed through the first bed, countercurrent to the
feed direction, to purge the bed at atmospheric (or low) pressure. After the purge,
the unit is ready for the next cycle. Thus each bed undergoes two half-cycles,
the times of which are equal. For a purification process, steady and continuous
flows of both feed and purified product are achieved.
Two useful intuitive ideas were also described by Skarstrom’s invention. First,
short cycles and low throughput per cycle should be maintained in order to “con-
serve the heat of adsorption.” This idea was derived from the observation that
long cycles and high throughputs would result in hot beds during adsorption
and cold beds during regeneration, both detrimental to separation. The second
idea was that the volume of both purge and feed, at their respective pressures,
should be at least equal to ensure complete purge, or displacement, during regen-
eration. These two basic ideas have been determined to be very useful in later
developments.