Page 95 - Design of Simple and Robust Process Plants

80 Chapter 4 Process Synthesis and Design Optimization
and examples include: conversion from gas phase to liquid phase reactions;
improved catalyst systems; combinations of reaction; and separation such as a zeo-
lite membrane reactor or reactive distillation. New reactor configurations, such as:
switching from packed bed to fluidized bed system (Dutta and Gualy, 1999) or a
homogeneous system, reverse-flow reactor systems (van de Beld and Westerterp,
1996); interstage component removal (Westerterp et al., 1989); development of a
bubble column reactors (Schluter et al., 1992) are all improvements that have been
explored. The results of this research are configured in reactor models. Improve-
ments in separation are also often subject to detailed research activities. On examin-
ing developments at the grass-roots level it should be mentioned that these also
focus on a particular catalyst system and configuration. The results of such research
activity are included in what is called ªfeasible flowsheet modelsº. Before the concep-
tual design with its synthesis study of a project is started, the results of research will
be evaluated in the context of other process routes in a feasibility study.
The alternative options for the sequencing of separations will remain, but many
processes have a limited number of separations, or have constraints which limit
freedom with regard to the number of options available.
The limitations in technical knowledge and the constraints justifies the conclu-
sion that improvements to an existing system or a grass-roots design often have lim-
ited practical alternatives to be introduced into a (preferably fundamental) model.
The number of alternatives is also restricted due too the application of the layered
approach-if this approach isn't applied the number would increase tremendously,
this is at best be reflected in a decision Figure 4.5. The limited number of alterna-
tives, and the ease of their simulation and overall design, makes the comparison of
alternatives on a direct, economic basis and the avoidance of large optimization
problems an attractive proposition. This does not mean that broad studies are not
carried out. Indeed, one benefit of a broad study is the identification of opportunities
for further developments. In the context of sustainability, a search for cheaper routes
by increasing knowledge and solving constraints will undoubtedly gather interest.

4.1.4
Economic Calculations

Economics ultimately determine whether a process will be executed, and a number
of factors play an important role in achieving an objective, optimal design:
. An economic evaluator and a professional cost estimator should be involved
in the design team to ensure objectivity in the cost estimates and evaluations
. The cost estimator is responsible for updating estimation data for the specific
location in the database used for the evaluation and optimization of process
design alternatives as part of the simulator. Discontinuities in the cost data
set for equipment must be smoothed in order to avoid converging problems
during optimization.