Page 377 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

Chapter 12 Synthesis of the PFD from the Generic BFD

The evolutionary procedure to create a full PFD (as presented in Chapter 1) from the generic block flow
process diagram (GBFD) (Chapter 2) is described in this chapter. This full PFD truly defines the process
in a chemical engineering sense and is the starting point for chemical and other engineers to design the

machines, structures, and electrical/electronic components needed to make the chemical engineer’s vision
a reality.

This crucial step in the design of the chemical plant involves all subareas of chemical engineering:

reaction engineering, thermodynamics, process control, unit operations and transport, and material and
energy balances. Each is applied to put details into the six general sections of the GBFD—reactor feed
preparation, reactor, separator feed preparation, separator, recycle, and environmental control.

In this synthesis, the broader context of the project (e.g., environmental concerns, customer expectations,
return on investment) is integrated with the important details such as the type of heat-transfer medium or
the number of stages in a column. It is crucial to consider as many alternatives as possible in the early
stages to try to avoid becoming trapped in a suboptimal design.

It is a common human trait to resist change more strongly as more effort is expended on a task,
design, or product. We describe this as not wanting to abandon our “investment” in the
activity.

12.1 Information Needs and Sources

Before the detailed synthesis of the PFD can be completed, one needs basic physical property and
kinetics information. We assume here that the very basic chemistry of the desired reaction is known, that
is, what main feed materials go to what main product. Before PFD synthesis can begin, the marketing
engineers have identified a market need for a specific product, and the chemists have identified at least
one way to produce the chemical in the laboratory. Even the marketing and chemistry information,
however, will need to be refined. Flowsheet synthesis will uncover the need for more detailed data on the
reaction rate, temperature and pressure effects, and market values of products of different purities.

12.1.1 Interactions with Other Engineers and Scientists

Teams of engineers work on the development of the process. For example, the marketing department will
find the customer for the plant’s product, and product specifications will be identified. Many chemical
engineers are employed as marketing engineers, and they will understand that product purity affects
product price, often dramatically so. However, the details of this interplay can be determined only by the
process design engineers as the PFD is being developed; only through discussions and negotiations with
customers can the marketing engineer determine the relationship between product purity and the product