Page 184 - Design of Simple and Robust Process Plants

5.5 Intensification of Process Functions 169
5.5.1.2 Heat transfer
During the 1970s, heat transfer was somewhat limited to plate frame heat exchan-
gers with a larger surface-to-volume ratio than shell and tube exchangers. These
units found wide application in the food industry to treat sticking and fouling
streams, but were used to only a limited extent in the chemical industry. Their
major drawback was that they were fitted with numerous large gaskets that had a
tendency to leak. The improvements expected during intensification (this was also
driven by the motor car industry, which had a need for reliable tight, low weight,
compact heat exchangers), were large area/volume, tight systems, multi-stream
units, and suitable for multi-phase applications. The leading vendors in this field
accepted the challenge, and initially the system was tightened by the use of semi- or
total brazing or welding techniques to limit/eliminate the need for gaskets. The next
step was to increase the surface area by introducing fin plate heat exchangers, or
even micro channel heat exchangers, to increase performance. This brought com-
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pact exchangers of 1000 M /M . The construction options of the device made appli-
cation as evaporators and condensers and also multi-stream designs a reality.
Enhancement of the heat transfer coefficient was realized by the application corru-
gated plates. The drawback of the exchangers is that applications are restricted to
nonfouling systems. The advantage, next to the size and low cost, is the perfor-
mance which makes designs with a small temperature approaches attractive. Thus,
higher energy utilization is to be effected, which is inclusively realized by the coun-
tercurrent operation. For one-phase systems this can conventionally be realized by
multiple shell and tube exchangers in series, though at very high cost (Thonon and
Mercier, 1997; Edge et al., 1997). The applications of compact heat exchangers are
quite extensive in the automotive industry, and in air-conditioning, refrigeration and
liquefied gas applications. Further applications at the process industry are at hand,
but ªtrend-settingº applications such as reboilers and condensers are needed to
advance this situation. Further application in this area will push the shell and tube
heat exchangers into the role of museum pieces.
Future developments might lead to applications such as reactive heat exchanger
for highly exothermic, rapid reactions, including those of catalytic plate reactors,
and developments are ongoing in this area.

5.5.1.3 Mass transfer
Mass transfer operations of gas±liquid systems urgently require size reduction.
Efforts in this direction, based on the use of centrifugal fields to enhance phase sep-
aration and mass transfer, have not been practicable on an industrial scale. Mass
separation by membranes offers another approach to reduce the size, and surface-
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to-volume ratios over 1000 M /M have been exceeded by far the ratios of conven-
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tional, direct contact gas liquid separators (typically 100 M /M ) (Jansen et al.,
1995). Gas absorption might be physical or reactive, and no differentiation will be
made between these. Membrane separations such as reverse osmosis, ultrafiltration
and microfiltration are now considered to be standard technology. For the operation
of membrane gas absorbers, the majority of conventional absorbents can be used,
although it is vital that the absorption liquid does not penetrate the membrane. For