Page 205 - Adsorption Technology & Design, Elsevier (1998)

Selected adsorption processes 191

meanwhile the majority of the nitrogen is adsorbed and retained in column
1. The third step occurs when the columns are again isolated and column I is
depressurized to atmospheric pressure (commonly known as blowdown)
causing nitrogen to be desorbed and flow from the bed (countercurrent to
the direction of feed in the second step). The last step is for the beds to be
reconnected and some oxygen (produced from the second step in the cycle)
is passed through both columns countercurrent to the direction of the air
feed. This latter step of the cycle ensures that any adsorbed nitrogen in the
bed is flushed towards the column entrance thus allowing the major portion
of the bed to be free of adsorbed nitrogen and ready for the whole cycle to be
repeated. Column 2 goes through a similar cycle of events to column 1
during a cycle. This process for air separation was developed by Skarstrom
(1960 and 1975) and is used for small-scale separation units. The original
patent was assigned to Exxon Research and Engineering in 1958.
A patent for air separation was also granted to L'Air Liquide in 1964. The
cycle was developed by Guerin de Montgareuil and Domine (1964) and is
known as the Guerin-Domine cycle. Three steps are involved for each of the
two beds. The first step is the pressurization of bed 1 while bed 2 is
evacuated. The second step is the downward blowdown of 1 through the
previously evacuated bed 2 from which oxygen is collected. The third step is
the evacuation of bed 1. The roles played by beds I and 2 become reversed
for the following cycle. Nitrogen is released from each of the beds 1 and 2
during evacuation. Compared with the basic Skarstrom cycle, the
Guerin-Domine cycle gives an improved performance because N2 is
removed efficiently by evacuation thus leaving a clean bed for the elution of
O2. The introduction of an evacuation step nevertheless increases the
expenditure of mechanical energy.

7.2.2 Improvements to the basic PSA cycle
Although the Guerin-Domine cycle proved to be more effective for the
separation of air than the Skarstrom cycle, the former cycle was not
economic. The main improvements over the basic two-bed cycle which have
occurred in the last three decades are the introduction to the cycle of
cocurrent depressurization and pressure equalization. Extension of the
number of beds in series and the sequence of operational steps in the cycle
have led to major process improvements.

Cocurrent depressuHzation
Incorporation of a cocurrent depressurization step immediately following
the pressurization and feed steps into the basic Skarstrom cycle increases the