Page 177 - Design of Simple and Robust Process Plants

162 Chapter 5 Process Simplification and Intensification Techniques
5.3.3.2 Extrusion
Historically, extrusion has always been used for a combination of physical functions.
Examples include mixing with other polymers, blending of additives, pumping and
product up-grading, as well as the stripping of polymers from volatiles. The strip-
ping technique, through the addition of a stripping agent, has received much
attention during recent decades. These techniques were increasingly applied when
environmental health requirements for food packaging regarding the removal of
monomers and other volatiles became more stringent. More recently, a combination
of reaction and extrusion has been applied in this respect (Table 5.3).

Table 5.3. Combination of functions in an extruder.

Extruder Application(s)
Reaction Co-polymerization
Product upgrading Heat treatment for product
Blending Coloring, additives as anti-oxidants
Pumping Push-through die
Heat exchange Heating and cooling
Stripping Removal of volatiles

5.3.4
Furnaces

Furnaces are used for heating purposes, and offer many opportunities for the com-
bination of functions. These are all well known, but are referred to here for comple-
teness.
It is known from energy and exergy analysis that it not enough to achieve high
efficiencies based on the net heating value. What is required is not only a high effi-
ciency based on gross heating value, but also a minimum of exergy losses. High
efficiencies based on gross heating values can only be achieved by taking advantage
of the heat of condensation of the flue-gases, as is practiced in some gas-fired fur-
naces. Minimum exergy losses are achieved by exchange of heat with a small tem-
perature differences between flue-gases and other streams; this is realized by step-
wise heat exchange. As mentioned above, it is clear that the design requires careful
optimization of the energy household of a furnace, see Figures 4.39 and 4.40. The
standard functionality for combustion equipment is:

. power generation by gas turbine;
. reaction heating
. high-temperature process heating;
. medium-temperature process heating; and
. low-temperature process heating.