Page 41 - Sustainability in the process industry

18 Cha p te r T w o

discharge can considerably reduce demand for freshwater and also
the amount of effluent generated by processing.
For these reasons, the success of HI has inspired researchers to
apply the Pinch and PI concepts to other areas—in particular, to
mass exchange networks (El-Halwagi and Manousiouthakis, 1989).
Wang and Smith (1994) developed a method for industrial water
networks as a special case of mass exchange networks (see Figure 5.1).
Their main objective was to minimize the consumption of freshwater
and the disposal of wastewater simultaneously by maximizing the
reuse of internal water, again exploiting the idea of recycling and
reusing valuable streams and materials in order to save resources
and reduce emissions. Wastewater can be further reduced by
applying additional techniques for water regeneration that enable
further reuse or recycling. For the case of a single contaminant,
translating the method of Pinch Analysis to water minimization is
straightforward: the water’s Composite Curve is used to construct a
plot of contaminant concentration versus contaminant load.
Extending the Water Pinch Analysis to multiple-contaminant
problems is a complicated and difficult procedure. The principal
issue concerns determining which contaminant to use on the Y axis
when plotting the Composite Curves. Several approaches have been
proposed. One option is to employ MPR, in which case Water Pinch
serves as a preliminary visualization tool (Doyle and Smith, 1997).
Foo, Manan, and Tan (2005) applied Water Pinch Analysis to
synthesize optimal water recovery networks for batch processes.
These authors introduced a numerical technique (time-dependent
water cascade analysis) that has the advantage of clearly depicting
the time-dependent nature of batch water networks. Majozi (2005)
also employed mathematical modeling—the mixed integer nonlinear
programming (MINLP) approach—to devise an effective technique
for wastewater minimization in batch processes. Here, too, wastewater
was minimized via application of reuse and recycle.

2.8 Benefits of Process Integration
Heat recovery targeting for HEN synthesis problems is based on
Composite Curves described in Section 2.4 (Linnhoff, Mason, and
Wardle, 1979). The Composite Curves plot is a visual tool that
summarizes the important energy-related properties of a process in
a single view (see Figure 4.7). It was the resulting recognition of the
thermodynamic relationships and limitations in the underlying
heat recovery problem that led to development of the Pinch Design
Method (Linnhoff and Hindmarsh, 1983), which is capable of
producing maximally efficient heat recovery networks. As already
discussed in this chapter, PI has been considerably expanded in
scope since these initial applications. It is now also used for HEN