Page 70 - Sustainability in the Process Industry Integration and Optimization

P r o c e s s I n t e g r a t i o n f o r I m p r ov i n g E n e r g y E f f i c i e n c y 47

m ˜ Q h h (4.2)
HE cold in,cold out,cold
˜
˜
Q HE U A T LM (4.3)
where Q [kW] is the heat flow across the whole heat exchanger, U
HE
2
−2
[kW∙m ∙°C ] is the overall heat transfer coefficient, A [m ] is the heat
−1
transfer area, and ΔT [°C] is the logarithmic-mean temperature
LM
difference. More information can be found in Shah and Sekulić
(2003), Tovazshnyansky et al. (2004), and Shilling et al. (2008).
4.1.2 Implementing Heat Exchange Matches
The heat exchange matches are often viewed as being identical to
heat exchangers, but this is not always the case. A given heat exchange
match may be implemented by devices of different construction or
by a combination of devices—for example, two heat exchangers in
sequence may implement a single heat exchange match. The
distinction between the concept of a heat exchange match and its
implementation via heat exchangers is important because of capital
cost considerations.

4.2 Basics of Process Integration

4.2.1 Process Integration and Heat Integration
A historical review of the field was given in Chapter 2. Initially,
attention was focused on reusing any waste heat generated on
different sites. Each surface heat exchanger was described with only
a few steady-state equations, and the thermal energy saved by reusing
waste heat led to reductions in the expense of utility resources. This
approach became popular under the names Heat Integration (HI) and
the more general term Process Integration (PI). In this context, HI
means integrating different processes to achieve energy savings.
Engineers realized that integration could also reduce the consumption
of other resources as well as the emission of pollutants. Heat and
Process Integration came to be defined more widely in response to
similar developments in water reuse and wastewater minimization.
4.2.2 Hierarchy of Process Design
Process design has an inherent hierarchy that can be exploited for
making design decisions. This hierarchy may be represented by the
so-called onion diagram (Linnhoff et al., 1982), as shown in Figure 4.2.
The design of an industrial process starts with the reactors or other
key operating units (the onion’s core). This is supplemented and
served by other parts of the process, such as the separation subsystem
(the next layer) and the Heat Exchanger Network (HEN) subsystem.