Page 133 - Design of Simple and Robust Process Plants

118 Chapter 4 Process Synthesis and Design Optimization

Q Q
1+2 tot
Q
Q 1 2
T
F ∆ T P T F

STEAM

T

Fig. 4.29. Heat integration between processes with a control
configuration to absorb disturbances by heat balance control.

way to deal with these disturbances, and immediate compensation for the disturban-
ces and de-coupling of interactions are ways to do this. Figure 4.29 shows an exam-
ple of compensation for disturbances for heat exchange between two sections. The
subject of controllability analysis is discussed in detail in Chapter 8.

Safety and environmental In Section 4.1.2, constraints were discussed from safety
and environmental points of view. Although HAZOP studies will draw attention to
these problems, they do so far too late in the design, and there will be a need for
add-on provisions. Particular attention must be given to streams that are directly
coupled with other processes, as any upset may lead to serious problems ± perhaps
unexpected impurities, a major change in pH, or an oil layer on water (or vice
versa). Special attention must be paid to heat-exchanging streams, as these have a
tendency to leak over time, potentially leading to problems of inter-reactivity
between streams. Special consideration must also be given to temperature-sensitive
materials, particularly under low-or no-flow conditions in heaters, as steam control
valves always leak and may cause problems of inter-reactivity or plugging. Unexpec-
ted situations may also occur with regard to environmental problems.
In conclusion, it might be said that concern exists regarding inter-process connec-
tions as a result of integration between processes. Potentially serious problems that
may occur during upsets are not limited to direct connections but also indirectly in
heat exchangers. In this respect, the design should be driven especially by inherently
safer conditions.

Economical Economical constraints speak for themselves, although often there is a
tendency not to take the full cost and realistic savings in consideration. The cost of
provisions for back-up, controllability, and operability are often not (or are insuffici-