Page 312 - Sustainability in the Process Industry Integration and Optimization

T y p i c a l P i t f a l l s a n d How t o Avo i d T h e m 289

steam generation line without making provisions for
preheating and superheating.
4. Extracting at the effective temperature. In some cases a stream
cannot be extracted directly because it has to still be used by
a related process. For example, a hot stream should be
extracted at temperatures at which the heat becomes
available. An example is given in Smith (2005, p. 433) for a
reactor using a quench liquid.
5. Forced and prohibited matches. In almost any process there exist
matches that are necessary for technological reasons—the
forced matches—as well as those, such as hot and cold stream
matches in a heat exchanger, that should be prohibited (e.g., to
prevent contamination of one of the streams). In manual design
these constraints have to be observed by the designer, but
software tools usually offer this option. If not then the
constraints can be secured by an appropriate penalty or bonus
(as applies) in the objective function used for the optimization.
6. Keeping streams separate only when necessary. If streams can be
merged then it may be possible to eliminate some heat-
exchanging units. For example, streams that leave the plant
to be treated as wastewater often have some heat content that
can be utilized.

12.1.5 How Can the Heat Loads, Heat Capacities, and
Temperatures of an Extracted Stream
Be Calculated?
Once a stream has been extracted, the next problem is calculating the
heat-related data. There are standard engineering procedures
available for the running plants as the measurements with the
following data reconciliation (Klemeš, Luťcha, and Vašek, 1979;
Minet et al., 2001; VALI III User Guide, 2003). The other option is to
develop a flowsheeting simulation model (Klemeš, 1977). An
overview of flowsheeting and balancing simulators was given in
Chapter 9. If a plant is being designed, some data could be also
extracted from the process flow diagram (PFD). But all those options
consume time and resources, so in the early design stages (when the
process structure is still under development and likely to be changed
as a result of PI analysis), it is reasonable and easier to use a simplified
approach based on the extracted data. Such an approach is
demonstrated in Figure 12.5 for a part of the flowsheet from
Figure 12.1. The CPs of the stream segments are assumed constant
and calculated from the temperatures and the duties given in the
flowsheet. Experience has indicated that the resulting rough
preliminary data are sufficient and can later be made more precise
by one or more of the procedures listed previously.