Page 99 - Design of Simple and Robust Process Plants

84 Chapter 4 Process Synthesis and Design Optimization
advantage that design engineers can follow the evolution of the optimal design and
understand the impact of design decisions. The design engineer must model all the
process alternatives in the simulation/optimization. By using this layered approach
he/she receives the intermediate results for evaluation. The optimization approach
as discussed has four successive layers, each of which will gradually increase the
size and details of the models and adapt the optimization (objective) function. The
number of alternative options is reduced as the decision tree is descended
(Figure 4.5), while the number of DOFs seen as relevant to the problem is rese-
lected.

Synthesis decision tree

Starting point

Reactions

Separations

Integration

Controllability

Optimization

Fig. 4.5. Synthesis decision tree for a process plant.

The synthesis is initially performed as per process sections (see Section 4.1.4). To
cope with the interaction between these sections it is necessary to optimize the inter-
mediate stream compositions and price setting, this can be done by a few manual
iterations in a synthesis step(mostly carried out during optimization layer 2 and 3).
Iterative calculations might be minimized by assigning a cost and capital value to a
recycle stream as was mentioned under the economic section. Ultimately, the com-
plete process must be optimized after combination of the sections, and this will be
done with a limited set of DOFs per section.

4.1.5.1 The first layer
The first layer relative to the reaction includes separation and integration synthesis
steps 1. The intermediate prices are determined (as discussed in Section 4.1.4). For
the energy prices, the site energy prices are taken initially at thermal energy levels
relevant to the application.