Page 307 - Sustainability in the Process Industry Integration and Optimization

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may over-estimate the potential benefits. Appropriate data extraction
accepts only the critical sections of the plant which cannot be
changed. Data extraction skill develops with increased experience in
the application of Pinch Technology (Linnhoff March, 1998).
Since the release of these packages, the methodology has
developed further and more attempts have been made to extract
data automatically. However, experience and following the proper
rules remain valuable assets. Basic questions to ask include the
following:

1. When is a stream a stream?
2. How precise must the data be at each step?
3. How can considerable changes in specific heat capacities be
handled?
4. What rules and guidelines must be followed to extract data
properly?
5. How can the heat loads, heat capacities, and temperatures of
an extracted stream be calculated?
6. How “soft” are the data in a plant or process flowsheet?
7. How can capital costs and operating costs be estimated?
12.1.1 When Is a Stream a Stream?
To those unfamiliar with PI, identifying a stream seems fairly
straightforward. In fact, many considerations are involved, and
accounting for them properly is key to setting up the problem. First,
we need not consider any stream that neither gains nor provides heat;
no data needs to be extracted from a stream with identical supply
and target temperatures and enthalpies. (Of course, in the absence of
perfect insulation, every stream loses or gains some heat; in many
cases, however, these small amounts of losses and gains can be
neglected.) If we do not extract data from such streams, the problem
is considerably simplified.
There are also streams that, for one reason or another, should not
be included in the PI problem—for example, streams that are remote
and streams that should not be altered for safety, product purity, and
operational reasons and for other (mostly practical) considerations.
Finally, Heat Integration deals with heat flows, which can be carried
not only by a pipe line but also by radiation or conduction.
Consider the example depicted in Figure 12.1. This example was
introduced, along with some data extraction rules, by Linnhoff and
colleagues (1982) and was later modified for use in many follow-up
books (e.g., Smith, 1995; Smith, 2005; Kemp, 2007) and in many courses
based on UMIST (later the University of Manchester) teaching
materials. The figure shows part of a flowsheet in which the feed
stream is heated to 45°C by recuperated heat in a heat exchanger and
then enters a processing unit. After leaving this unit, the stream is