Page 442 - Design of Simple and Robust Process Plants

10.3 The Design of High-quality Plants 429
Timing/documentation: The brainstorming session results need to be documented by
the team, the ideas selected for further development, and implementation needs to
be reported by the project team. The process simplification is done at the beginning
of the conceptual design with the first-pass mass balances available.

10.3.2.4 Process and energy optimization
The objective of the VIP is to optimize the design of a facility as a trade-off between
capital and operational costs. This VIP was originally limited to energy optimization,
but has been extended with process optimization. The reason is that the impact of
process optimization on an efficient design is much larger than from energy optimi-
zation alone. Historically, the situation can be explained, the technique for energy
optimization with its pinch approach was developed immediately after the energy
crisis at the end of the 1970s. The technique for design optimization is industrially
(in all practicality) applied at the turn of the century.
Optimization of design has, in comparison to operation optimization, a much
larger number of degrees of freedom. Designers want to understand the pathway of
process decisions, and therefore plant optimization for design is split into different
layers (see Chapter 4). The layers to be mentioned are:process synthesis; optimiza-
tion of consolidated flowsheet including energy optimization. In the long term,
there will be developments in commercial software to make integration of these dif-
ferent optimization levels possible. However, many practical problems will remain
to be overcome before process designers apply this technique with full confidence of
their designs. Remember the process simplification effort which is still at the begin-
ning of an evolutionary state. In a full design optimization model, a large expert
system needs to be available to generate all feasible alternatives. Some time will pass
before that is realized. For revamps ± which still represent the majority of the pro-
jects ± this will certainly not be solved in the short term. There is no doubt that,
ultimately, there will be developments to combine the integration of these optimiza-
tions levels.

Conceptual design optimization or process synthesis
As the word says, this must be carried out during the conceptual design. It might be
performed by evaluation, optimization, of different cases against the objective func-
tion. An alternative way is to describe it as a mixed integer problem based on a
super structure with its constraints. In Chapter 4, this level was split over two syn-
thesis steps, the final result being an overall flow diagram with the type of separa-
tions with its sequence selected and preliminary mass balance ± called a ªconsoli-
dated flowsheetº.

Optimization of consolidated flowsheet
The overall flowsheet has been fixed, but the pressure levels, the specification of
recycles, and reactor optimal design point are still free, as are the equipment dimen-
sions. The techniques applied are mixed integer or continuous optimizations. The
methodology for continuous flowsheet optimization is described by Koolen et al.