Page 62 - Design of Simple and Robust Process Plants

46 Chapter 3 Design Philosophies
conceptual design of a process plant will ultimately not be solved as one problem,
on the basis that:

. The alternatives for the design will still be subject to the inventiveness of the
designer. There is no doubt that the quality of the synthesis tools including
generation of design alternatives will improve, but (as learns the examples
for simplification in Chapter 5), there is still a long way to go.
. Designers always want to keep track of the decisive steps in an optimization
effort. They tend to follow a step-wise approach, with intermediate validation
of results and reconsideration of active constraints.
. Optimization software is currently not sufficiently robust to solve major
optimization problems, nor to include the effects of integration and controll-
ability.
. Designers will (in agreement with business) always try to minimize the risk
of a design, and therefore often take a more conservative route. This does not
mean they do not want to quantify the best design ± but they may decide on
another route.
Based on the above conclusion, process synthesis have to be carried out at different
hierarchical levels. The detailed levels that are discussed follow an interactive onion
model, which is based on the conventional onion model of Smith and Linnhoff,
1988.
Between these different hierarchical levels, interaction and process simplification
need to take place in order to optimize the total plant design, as is illustrated in
Figure 3.3.
The optimization will be done in a layered approach. It starts with the evaluation
of many process alternatives. The number of alternatives will gradually be reduced
while the modeling details will enlarge. Ultimately one overall process flowsheet

Site integration
Utilities Controllability
Heat exchange Integration
Separations/ Separations/
Recycle Recycle
Idea generation
Reactor Reactor Optimization
Interaction

Conventional Interactive

Fig. 3.3. The interactive onion model versus the conventional onion model.