Page 220 - Design of Simple and Robust Process Plants

5.7 Simplification and Ranking per Unit Operation 205
mance of continuous operations. In particular, the density gradient over the contin-
uous phase will support or depress any overall circulation in this phase.
The simplest extraction will be one stage, and therefore start the ranking:
1. One-stage mixer settler; here, the mixer is selected as a static mixer, being the
simplest component. On occasion, rotating mixers are used, but these are
mainly applicable at large flow variations. In that respect, a pump is not a
very good mixer as it is not designed for that purpose. The settlers selected
will be gravity-based, and provided with additional surface area by the instal-
lation of separation plates located short distances apart to decrease the set-
tling distance and time (see Figure 5.24). These separation plates also limit
any overall circulation that will reduce the performance. A disadvantage of
the system is the large hold-up and related residence time.
2. Spray columns are used for larger flow rates, but have the disadvantage of
overall circulation. This causes a deterioration in performance such that only
one or two theoretical stages are achievable.
3. In packed columns, the packing reduces overall circulation, but in the case of
counteracting density differences will not be sufficient.
4. Tray columns.
5. Mechanically agitated columns; several alternatives have been developed over
the years, but with limited performance differences. Included among these
are rotating disc columns, the Kühni column with rotating impellers, and
Karr columns provide with reciprocating vertical displacement of stacked
discs.
6. Pulsing columns; the feature is a reciprocating device like a piston or mem-
brane, which moves the liquid to enhance mass transfer. This is often imple-
mented as an additional device to a packed or tray column.
7. Centrifugal extractors ( Podbielniak extractors are a specific alternative);
these are intensified units with multi-stage performance. They are useful for
small density differences, and have little hold-up.
Note: multiple stage mixer settlers have been eliminated as being a bad choice.

5.7.4
Adsorption/Chemisorption

Adsorption is performed, as a first choice, batch-wise in fixed beds, and two parallel
beds are often installed for alternate use during regeneration. The benefits of this
approach are the ease of operation and robustness in performance. Regeneration
makes this operation rather expensive, as either aggressive chemicals as used for
ion-exchange applications or conditions of extreme temperature and/or pressure are
required. The most common application are ion-exchange units for water treatment.
New techniques have been developed to reduce the size and ease of regeneration.
This technique is based on frequent switching (within minutes) between adsorption
and de-pressurized desorption (regeneration). By doing so, the heat of adsorption is
available for desorption. This technique of high-frequency pressure swing adsorp-