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5.3 Combination of Process Functions 147
The reduction of carrier streams (e.g., water) can be avoided by the elimination of
make-up water and direct steam injection in a process using steam ejectors or steam
strippers, or water washing without in-process water recovery. An example of this is
where a process has a multi-stage air compressor with interstage cooling; the water
condensed is considered as waste, as potentially it might be polluted with oil-seal
liquid. This water might represent a good source for make-up water, and as such
reduces the waste load.
In general terms, the collecting of all kinds of waste streams for treatment should
be avoided. A better approach is to set up internal recycle provisions that are specific
to each stream and return these to the process at appropriate points.

5.2.3.1 Do-undo-redo Activities
These are often performed during a process, and might include heating-cooling-re-
heating. Other examples are pressurizing-de-pressurizing-pressurizing, or separa-
tion-mixing-separation. To trace these situations a line diagram can be prepared that
shows temperature, pressure, and concentration over the major processing routes,
see Section 4.2.3.3 in Chapter 4. This representation can easily trigger do-undo-redo
situations and offer a challenge for a surge in potential improvement. These activ-
ities may be considered for the elimination of equipment such as pumps or exchan-
gers, as well as the elimination of recycle provisions to prevent mixing of streams of
different compositions and/or temperature that always lead to a loss in exergy.

5.2.3.2 Single trains and installed spares
Single trains will be discussed under overall process simplification (see 5.6.2). The
approach regarding installed spares will be discussed under the topic of reliability in
Chapter 6.
Separations are often more difficult to avoid, but combination with other separa-
tion functions are discussed in Sections 5.3.2 and 5.3.3.

5.3
Combination of Process Functions

The combination of process functions into one unit has been extensively developed
during the past few decades of the twentieth century.
The combination of different process steps in batch-operated vessels, for example
mixing, reaction, devolatilizing, neutralization, stripping, cooling, crystallization,
and feeding additions, is the oldest form of performing process functions in a single
piece of equipment. These batch operations are typically found in the food and drug
industries, as well as in the fine chemicals industry (see Figure 1.4 in Chapter 1).
The multi-functional use of equipment does not fall under the banner of ªcombina-
tion of functionsº, as described later. Although it should be noted that multi-func-
tional use of equipment has its merits, we will discuss here the types of combina-
tions that lead to simpler designs in continuous processes (for a review on multi-
functional reactor systems, see Westerterp, 1992).