Page 64 - Design of Simple and Robust Process Plants

48 Chapter 3 Design Philosophies
Process integration can be differentiated as follows:

. Direct integration between process units with process streams.
. Integration between process units on ªutilitiesº levels, such as energy and
water.
. Integration with utility plants, for example in plant power generation.
Hardware de-coupling is illustrated in Figure 4.28. A direct cross-heat exchanger
between two process sections can be de-coupled by installing two additional heat
exchangers ± a back-up heater, and a cooler. Integration between units at utility lev-
els is shown by replacing the process cross-heat exchanger by two exchangers with a
utility steam back-up. The hardware de-coupling mainly tackles the availability.
For integration within the same process section direct exchange is often applied.
Integration between process sections of the same plant is quite frequently done by
exchange at plant utility level. Integration between different processes is preferably
done through exchange at site utility level.
Software de-coupling is illustrated in Figure 4.29. In this example of a distillation
column, the heat produced from an external source passes to an exchanger that is
operated in parallel with an existing steam reboiler that can take the full load. By
measuring the total heat duty of the exchangers and manipulating that heat duty for
control purposes, the systems are de-coupled and compensation is made for external
disturbances. A similar solution can be applied to the source side. In this way, it is
possible to cope with the availability as well as the disturbances.
The problem of availability and vulnerability is a major issue for a chemical com-
plex, particularly if the level of integration is increasing (Koolen et al., 1999). Optimi-
zation of the design of an integrated chemical complex with regard to availability
and unit/service reliability will be discussed in Chapter 7.

Summary
. Process integration is increasing at all levels, with progressive pressure to
improve performance in terms of raw materials and energy utilization.
. Process integration may conflict with simple and robust plants, and must be
addressed by a higher level of automation and control.
. The major concerns of process integration are availability and disturbances.
. Back-up provisions are often used to cope with availability during outage or
malfunction of integrated units. In general, integration between processes is
carried out at site utility levels in order to provide back-up.
. Disturbances are rejected through the introduction of advanced control tech-
niques.
Optimization of the design and operation of chemical complexes needs to include
an availability study based on the reliability of the units/services.