Page 94 - Design of Simple and Robust Process Plants

4.1 Process Synthesis 79
Before a synthesis study is commenced, an inventory of the constraints must be
made; moreover, extensive testing may also be needed in order to avoid surprises at
a later date. Although the constraints on a process synthesis study are considerable,
it is possible that by recognizing such constraints, engineers are sufficiently intelli-
gent to devise clever solutions during process synthesis.

Summary
. Constraints have a major impact on the degrees of freedom for a process syn-
thesis study. These constraints can be subdivided as: physical and chemical
properties, safety, environmental decomposition, fouling, corrosion, utility
and controllability.
. The hazards of a components or mixture and its incompatibility as present in
the process must be collected, documented, and subjected to evaluation.
. The hazards of these components need to be used as input for the application
of the inherently safer design principles during the first layers of the process
synthesis and to define the constraints of the design and operation.
. Environmental requirements can be divided into emission minimization and
sustainability. Both are subject to moving targets set by Society, but are
increasingly directed by global concerns and regulations.
. Sustainability is a criteria for the selection of the process, and will impact on
the development of new process routes. Sustainability also drives for higher
process efficiencies that, next to improvements in process synthesis, also lead
to further integration between processes.
. Emission minimization should preferably be achieved by more selective reac-
tor/separation designs, eventually followed by efficient abatement techniques.
. Internal recycling is applied to aim at lower emissions. This leads to build-up
of impurity levels, which demand a careful design of the separation system
to achieve an optimal process. A mass balance covering all impurities, includ-
ing formation, build-up and removal, is required for a good design.
. Decomposition reactions and fouling often place constraints on the proces-
sing temperature, but the wall temperature might also play an important
role. Recognition of these constraints is important in order to avoid excessive
cost in operation.
. Corrosion problems and utility restrictions are not necessarily constraints,
but in practice they often exert a high economic penalty on certain synthesis
options.

4.1.3
How Broad is a Synthesis Study?

A synthesis study is often started during and/or after the execution of a R&D project
aimed at improving an existing process. These developments were triggered by an
idea to improve a reactor system by developing an alternative catalyst system or reac-
tor configuration. They focus on improvements of selectivity's and/or conversion,