Page 152 - Design of Simple and Robust Process Plants

4.2 The Methodology of Process Synthesis 137
Ultimately, the control configuration is selected. The controllers need to be de-
signed, and can be tested in the dynamic simulation. Although this is part of the
control design, it is outside the scope of the process synthesis.

4.2.5
Flowsheet Optimization

The input of the optimization is the selected flowsheet after controllability step 2.
The objective is to deliver an optimized overall plant design, including all major
equipment dimensions and mass±energy balances. The design will be input for
basic engineering, to specify equipment, and to develop piping and instrument dia-
grams and its specifications. The optimization activities have been described under
layers three and four of the optimization methodology (see Section 4.1.5). The third
layer sets the design, while the fourth layer of optimization confirms the overall
design, and particularly the sensitivity of the design for price variability. A deliver-
able of this layer includes a basis for an operational optimization model to be used
for operation optimization.
The optimization is concluding the design as developed during the synthesis
steps. Any simplification activity should have been incorporated in the design before
this flowsheet optimization is executed. Thus, the quality of the optimization is reli-
ant on:
1. business data (price sets);
2. the quality of the synthesis steps;
3. the accuracy of the model development;
4. the robustness of the models; and
5. DOFs made available for optimization.
The quality of a process is always under evolution through: clever thinking; evolving
technology; and business and environmental conditions. The quality of the design is
only valid within the assumptions made for the design. However, as these assump-
tions and conditions will change, monitoring of the quality of the design is impor-
tant. One way to do this is to search for incremental improvements ± this can easily
be done with off-line optimization runs where the effect of removal of constraints
can be quantified (see Chapter 10). Updating of the model is essential to achieve
optimal operation optimization and design improvement identifiable.

4.2.6
Logistics and Site Integration

Logistics and site integration are two aspects which have an impact on the process
design that may be considerable. The evolution of a batch plant is shown in Figures
1.4±7 in Chapter 1. It can be seen that the design evolved from one where most of
the handling was included. Nowadays, many of the supplies and services are pro-
vided from external suppliers at required conditions.