Page 101 - Design of Simple and Robust Process Plants

86 Chapter 4 Process Synthesis and Design Optimization
4.2.1. To minimize the optimization effort the number of DOFs for the alternatives
are minimized to those which have a major impact. The operating area is also
bracketed, though care must be taken that it is not narrowed too much.
For the separation section the inputs are the selected separations and the preli-
minary sequence of separations. Now, constraints are defined based on the safety,
environmental data and the static controllability studies. A preliminary sequence
and constraints evaluation eliminates those alternatives that are far from obvious.
The next step is to generate a complete list of remaining alternatives, including inte-
gration alternatives based on the target study, for further evaluation on NPV. The
process simplification options need to be introduced at this stage. The number of
design and operational DOFs is limited by arbitrarily selecting pressure and separa-
tion specification for those streams which are determined economically. Compari-
son of the optimized alternatives brings us into the position of selecting the best
competing flowsheets. At this point, preliminary equipment sizes are known as well
as the DFC and NPV.
Integration in this layer is repeated, but limited to the selected best flowsheets.
The constraints need to be revisited and further defined, such as utility conditions
and back-up provisions in network situations. The target studies will be extended
with the targets for heat exchange areas and number of units. The selected flow-
sheets will be subject to sensitivity (flat or steep optimum) and variability studies
regarding price sets of product and energy. The final selection for the flowsheet is
then carried out.

4.1.5.3 The third layer
The third layer of calculations during the flowsheet optimization synthesis step is
performed section by section, with updated intermediate stream compositions and
prices. The prices for thermal energy might be different depending on the tempera-
ture/pressure level. These prices can be determined in relation to the primary
energy price by calculating the net effect on primary energy consumption by incre-
mental or decremental heat consumption at the specific level. At this point of the
design, it is important to know the process integration in relation to the site energy
system.
Modeling is preferable carried out rigorously, but mixed integer problems should
be avoided (see Section 4.1.5.2). It should be noted that, for example, the feed point
selection for the distillation is not optimized during this layer, but this will be done
during the detailed process design. Now, all relevant operational and design DOFs
are set free, including pressure and separation specifications, while all constraints
must be reinspected. Some DOFs may be fixed in order to simplify the problem. In
the case of a distillation tower, at optimization layer 2 the reflux to minimum reflux
ratio is set free in layer 3 these might be fixed. At layer 3, when the energy prices are
the same and the specification is close to its layer 2 assumptions, these might be
fixed. The same applies for a reactor system, when for example the reactor pressure
and catalyst volume had no significant effect on the NPV, it might be decided to fix
these at layer 3. The selection of the relevant DOFs can best be supported by a sensi-
tivity analysis and its effect on the NPV. In case of any doubt, the DOFs should be