Page 221 - Design of Simple and Robust Process Plants

206 Chapter 5 Process Simplification and Intensification Techniques
tion (PSA) is applied for air purification. The desorption is carried out by depressur-
izing the system to vacuum, and has as an advantage that the bed size is largely
reduced. This in comparison with temperature swing absorption, which requires a
longer cycle time. An alternative adsorption system in use for hydrogen purification
is based on a simulated moving bed. A number of beds in series is changed during
service in a planned manner to simulate a continuous adsorption unit, with de-
sorption being instigated by depressurizing. The design of a special multi-port valve
has made this technique highly successful.
The design of a real, continuous adsorption system has found little application,
mainly because problems with solid handling were not solved adequately.
The order of ranking with increasing complexity has the following appearance:
1. Mixed ion-exchange beds to accommodate simultaneous removal of anions
and cations.
2. A high-frequency, alternating operation of absorption, desorption cycle by
depressurizing (PSA) is applicable only to gas systems.
3. Fixed bed with pressure, temperature or concentration desorption.
4. Simulated moving bed adsorption.

5.7.5
Heat Exchange

Heat exchange is one of the main supporting units for reaction and separation sys-
tems, and developments have concentrated successfully on more compact designs
for all types of application. Penetration of heat exchange concepts into the chemical
industry ± with its conventional shell and tube designs ± is still limited compared
with that into the food and pharmaceutical industries. However, penetration into
the chemical industry will clearly increase as the advantages of high surface-to-vol-
ume ratios and more plug flow operation associated with smaller temperature
approaches become apparent. The major limitation for chemical systems is fouling,
and it is imperative that this has to be solved from the processing point of view. The
list of heat exchange systems, in order of increasing complexity, is considered to be:
. Plate fin exchanger
. Plate exchangers
. Jacket pipe and jacket vessel
. Fin tube air coolers
. Hairpin exchanger
. Shell and tube exchangers with;
± fixed tube sheet
± floating head
± bayonet
. Mixed condensers; these are applicable for large quench towers where
quench material is kept in a closed system. Such a system still requires a
heat exchanger to remove the heat from the quench medium.