Page 85 - Design of Simple and Robust Process Plants

70 Chapter 4 Process Synthesis and Design Optimization
4.1.1
The Hierarchical Structure for Conceptual Design

The hierarchical structure for the conceptual design process is an extension of the
structures proposed earlier by (Douglas, 1988; Smith and Linnhoff, 1988; Douglas
and Stephanopoulos, 1995; Biegler et al., 1997; Seider et al., 1999).
The approach to the synthesis of a process is based on the interactive onion
model and presented for a known technology area. This is a quite common situation
in industry, the scope of the project being set by the business based on the market
situation. It includes projected capacity, product slate, product qualities, and technol-
ogy selection. A technology selection is the first technological step in a project, the
selection being a combined effort between business, R&D, and process technolo-
gists. The technology selection is based on technology that is available ± either com-
mercially or within the company. During technology selection, decisions concerning
the basic process route and the batch-versus-continuous process question are taken
in agreement with the business (for details, see Chapter 10). The starting situation
for a process synthesis study is, in addition to selection of the technology, the out-
lining of a feasible flowsheet, together with reaction and reactor information in a
modeling format that has been developed during the research phase. Both types of
information are very useful during the first approach of the process synthesis. Al-
though these initial designs are not optimized, they do generate some initial data
for conversion and recycling costs for unconverted reactants, as well as capital costs.
These data can be used for the initial reactor evaluations, which will be updated as
soon as the separation train has been synthesized.
The hierarchical structure has the following layers:
1. Reaction, selection of configuration.
2. Separation, selection of types and sequence.
3. Integration, types and level of integration.
4. Controllability analysis, static, dynamic.
5. Flowsheet optimization, of sections and the overall process.
6. Logistics and site integration.

The conventional approach of process synthesis is pictured in Figure 4.1.A, which
shows a sequential design while the interaction between the different layers is
shown as reverse directed dashed arrows. The hierarchical approach developed and
presented in this chapter is shown in Figure 4.1.B, with a more detailed presenta-
tion is presented in Table 4.1. This concept differs in; the introduction of a two over-
all development steps, which are separated by an idea generation step. The different
layers shown in two sequential overall steps indicate the gradual development and
interaction between the different synthesis layers. The interaction shown between
the layers within the same step is indicated by dashed lines while the overall flow
line is shown as a solid line.
Idea generation is shown as a separate activity between the overall steps in order
to emphasize the importance, and the best place for implementation. The idea gen-
eration particular addresses the introduction of process simplification techniques