Page 108 - Design of Simple and Robust Process Plants

4.2 The Methodology of Process Synthesis 93
where a practical pressure limit becomes effective, such as for vacuum sys-
tems, temperature for reactions with low energy effects and low activation
energy. The intention is to limit the size of the optimization problem
. The DOFs might be the ratio of reactants, ratio of diluent to reactants, conver-
sion, inlet temperature, pressure and reactor configuration.
. The outputs are the selected reactor type of CSTR/PFR or combination, with
the restricted optimized conditions by the selected DOFs.

4.2.1.3 Reactor simplification
The starting point for this activity is the research data for a certain reactor and cata-
lyst type described in a model. The objective is evaluation of the most promising
reaction configuration. The evaluation will initially be performed based on the
assumed conversions as applied in the feasibility flowsheet. The generation of alter-
natives with an emphasis on simplicity is now crucial. The following points for sim-
plification will be discussed and illustrated with some examples:
. Simpler reactor configurations
. Larger-scale reactor systems
. Combination of reaction and separation

Simpler reactor configurations This may be illustrated with some typical industrial
examples of simplified rector systems from a configuration perspective:
1. Replacement of a loop reactor with six exchangers by a boiling reactor (Fig-
ure 4.6); this resulted in a high capital saving, while the selectivity and con-
version were unchanged.
2. Replacement of three isothermal CSTRs in series by an adiabatic CSTR and
an adiabatic plug flow (Figure 4.7). This concept also led to high capital sav-
ings with removal of the recycle heat exchangers and six circulation pumps,
and the replacement of three vessel by one large vessel. The system was real-
ized by a slight increase in one of the reactants. The lower conversion
achieved was already over-compensated by energy recovery and a higher
selectivity.
3. The modification of a series of CSTRs for nitration reactors by an adiabatic
mixing street (Figure 4.8) (Hauptmann et al., 1995).
4. Other examples include the installation of multi-stage fluidized beds or a
riser bed to replace fixed or fluidized beds, or the replacement of a series of
CSTRs for gas±liquid reactions with multi-stage bubble reactors (Schluter et
al., 1992).
5. The installation of reverse-flow reactors (also called bi-directional reactors)
equipped with packing as heat exchanger, for adiabatic auto-thermal opera-
tion (Matros and Noskov, 1988; van de Beld and Westerterp, 1996; Kuczynski
et al., 1987) compared with a packed bed with cross-exchangers (Figure 4.9).
Several multifunctional reactors are descibed by Westerterp 1992.