Page 138 - Academic Press Encyclopedia of Physical Science and Technology 3rd Chemical Engineering

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Encyclopedia of Physical Science and Technology EN002G-100 May 19, 2001 18:49

760 Chemical Process Design, Simulation, Optimization, and Operation

FIGURE 5 Decomposition of process design into related tasks. (Adapted from Westerberg et al., 1979.)

1. Sequence of Distillation Columns components are similar. More recent work has improved
the selection of sequences for nonideal systems, such as
Initial work in separations synthesis involved the best se-
those that form azeotropes. Finally, other sets of heuristics
quence of distillation columns to separate an n-component
are available for the broader problem that includes ex-
mixture. For example, consider the two possible distilla-
traction, crystallization, membranes, or chromatography-
tion configurations to separate a three-component mixture
based separations processes.
(A, B, C ordered from light to heavy), shown in Fig. 6. The
first, known as the ”direct” sequence, yields a high-purity 2. Heat Exchanger Network Synthesis
distillate product of the light component, followed by a
column separating the two heavy components into high- Process plants normally have many streams that must be
purity streams. The alternative is to have a high-purity heated and many other streams that must be cooled. The
bottoms product stream from the first column, followed heat-exchanger synthesis problem is to find a set of heat
by a split of the light components into two high-purity exchangersthatminimizethetotalannualoperatingcostof
streams. Clearly, the number of possible sequences in- the plant. In the past, dedicated utilities were used (steam
creases dramatically with more feedstream components. for heating and cooling water for cooling), which resulted
The heuristics developed to rapidly screen for the best in systems that were easy to design and control but ex-
sequence work very well for ideal systems, where the pensive to operate. A major driving force for increased