Page 129 - Design of Simple and Robust Process Plants

114 Chapter 4 Process Synthesis and Design Optimization
. Selection of separations follows the heuristic rule that heterogeneous separa-
tions are preferable, and should be applied before homogeneous separations
The choice of phase separations is self-directing, and depend heavily on the
particle/droplet sizes
. Homogeneous separations can only be performed by the addition or creation
of another phase
. Heuristic rules are given for homogeneous gas and liquid separations. If the
selection of separation technique is not clear, there is no alternative for a cost
comparison.
. The most often used separation techniques for homogeneous gas and liquid
systems are distillatory separations as distillation, absorption, and stripping.
These can be easily (and reliably) designed from equilibrium data, while
sharp separation is achievable in one unit. The disadvantage is that capital
and operational costs are high.
. The generic superstructure for separation (Douglas and Stephanopoulos,
1995) was found to be widely applicable when compared with several indus-
trial processes.
. The pre-selection of distillation sequence during step 1 is generally made by
evaluating all alternatives that satisfy the constraints. Evaluation can be based
on the criteria of minimum vapor flow.
. Simplification examples were given for separation trains. In one example, a
five-column separation train was reduced a two-column concept. This was
realized by the installation of divided wall columns and dephlegmator tech-
nology. Simplifications represent the integration of functions and units (see
Chapter 5). Simplification is needed before step 2 of the synthesis work
. Optimization of the separation sequences is performed on full economics
during step 2, but with a selected set of DOFs.
. The selection should be limited to two or three flowsheets, which are further
developed during process integration step 2.

4.2.3
Process Integration

The final deliverables of process integration (Smith, 1995; IChemE, 1997) are select-
ed flowsheets which due to a higher level of integration are more efficient and
major equipment dimensions for the further evaluation of controllability.
Process integration is a development which is aimed at increasing raw material
and energy utilization of a design, and was initially driven by the energy crisis of the
1970s. Nowadays, these are even further intensified and seen to be much broader by
the need to create a sustainable world, mainly by further reduction of the consump-
tion of natural resources. Process integration may lead to complex operations which
require careful design. We can differentiate process integration as: