Page 88 - Design of Simple and Robust Process Plants

4.1 Process Synthesis 73
inherently safer design techniques. The concept is based on the assumption that
designers are not capable to generate all potential solutions right at the start of a
project. Designers will remain creative and generate new alternatives during the
design process and therefor an essential part of the proposed methodology is to
include a place for evaluation and reconsideration of alternatives. Simplification and
inherently safer design are an important part of the idea generation next to identifi-
cation of alternative units and configurations.
The fact that the layers are shown in a sequential order illustrates the decision-
making process. In reality, there will be many parallel developments to speed-up the
conceptual design, including:

. Selection of separation technique in step 1 will take place parallel to reactor
development in step 1.
. Controllability 2 will take place parallel with the other development in step 2;
the time-consuming step for development of dynamic models would other-
wise delay the progress.
. The parallel effort is justified, although the final conclusions about controll-
ability 2 can only be drawn after integration and equipment in preliminary
sizing are finished and controllability analysis is finished.
It was said before that next to idea generation and interaction, optimization was an
essential part of the synthesis process. The optimization effort is performed at each
layer and step, while the interaction between the layers is reflected in the economic
values for the intermediate streams which as such will have an impact on the com-
position of the intermediate streams. Interaction between reaction, separation and
integration should converge to an overall optimized design. The optimization is
approached in four layers which are over laying the synthesis hierarchy and is
shown in Figure 4.4 The optimization will start with the evaluation of lots of alter-
natives and shrink gradually to one final flowsheet where process conditions and
equipment dimensions are simultaneously optimized. The optimization
methodology be will discussed in more details in the Sections 4.1.4 and 4.1.5.
Controllability is split into static step1 and dynamic step 2 as early feed back to
the designer about any controllability problem is essential.
Flowsheet optimization is seen as a separate layer, an activity after the synthesis
has been completed and stream compositions and equipment dimension are tuned
to a final optimal flowsheet.
Logistics and site integration are the complementing activities to achieve an opti-
mal integrated process at the site.
The overall activities in the layers ± there is no split made between the activities in
the two separate steps ± are described as follows:
. Reaction
± Evaluate chemistry and raw materials, reactants purity
± Evaluate potential hazardous/toxicity of chemicals storage and handling
± Evaluate reactor types and conditions, the excess of reactants and the options
to shift the equilibrium