Page 400 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

The format for the flow summary table is given in Chapter 1. Each of the conditions (temperature,
pressure, flowrate, composition, and phase) should be estimated early in the flowsheet development. All
are needed to get preliminary costs, for example. Even estimates based on perfect separations can
provide sufficient data to estimate the cost of a recycle versus the value of burning the impure, unreacted
feed material as a fuel.

Completeness of the estimates, not their accuracy, is important at this stage. For example, an early
determination of phase (solid versus liquid versus gas) is needed to help choose a separation scheme or
reaction type (see Table 12.1).

12.9 Major Equipment Summary Table

Chapter 1 explains the requirements of a major equipment summary table. In the context of initial PFD
development, the process of creating the table forces the process design engineer to question the size (and
cost) of various units for which there may well be other options. If, early on, one must specify a large
compressor, for example, the process can be changed to a lower pressure or it can be modified to use
liquid pumping followed by vaporization rather than vaporization followed by compression. The early
consideration of materials of construction provides the clue that normal temperatures and pressures
usually result in less-expensive materials.

12.10 Summary

The inclusion of enough detail and the freedom to look at the big picture without the burden of excessive
detail are the keys to successful PFD synthesis. One must remain fully aware of the broadest goals of the
project while looking for early changes in the structure of the flowsheet that can make significant
improvements. Particular attention must be paid to the separations and reactor sections. The formation of

azeotropes between components to be separated greatly affects the separation sequence. These must be
identified early in the synthesis.

The beginning of the process is the generic block flow process diagram. Although the flow summary table
at this stage is based on crude assumptions and the equipment summary table is far from the final
equipment specifications, they help keep the chemical engineer cognizant of key choices that need to be
made.

References

1. Tables of Chemical Kinetics: Homogeneous Reactions, National Bureau of Standards,
Circular 510, 1951; Supplement 1, 1956; Supplement 2, 1960; Supplement 3, 1961. Now
available as NIST Chemical Kinetics Database at www.NIST.gov.