Page 77 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

them back to the reactor feed preparation block. Normally, the only equipment in this block is a
pump or compressor and perhaps a heat exchanger.
6. Environmental Control Block: Virtually all chemical processes produce waste streams. These
include gases, liquids, and solids that must be treated prior to being discharged into the
atmosphere, sequestered in landfills, and so on. These waste streams may contain unreacted
materials, chemicals produced by side reactions, fugitive emissions, and impurities coming in
with the feed chemicals and the reaction products of these chemicals. Not all of the unwanted
emissions come directly from the process streams. An example of an indirect source of
pollution results when the energy needs of the plant are met by burning high sulfur oil. The
products of this combustion include the pollutant sulfur dioxide, which must be removed before

the gaseous combustion products can be vented to the atmosphere. The purpose of the
environmental control block is to reduce significantly the waste emissions from a process and
to render all nonproduct streams harmless to the environment.

It can be seen that a dashed line has been drawn around the block containing the environmental control
operations. This identifies the unique role of environmental control operations in a chemical plant
complex. A single environmental control unit may treat the waste from several processes. For example,
the waste water treatment facility for an oil refinery might treat the waste water from as many as 20
separate processes. In addition, the refinery may contain a single stack and incinerator to deal with
gaseous wastes from these processes. Often, this common environmental control equipment is not shown
in the PFD for an individual process, but is shown on a separate PFD as part of the “off site” section of
the plant. Just because the environmental units do not appear on the PFD does not indicate that they do not
exist or that they are unimportant.

Each of the process blocks may contain several unit operations. Moreover, several process blocks may be
required in a given process. An example of multiple process blocks in a single process is shown in Figure
2.4(b). In this process, an intermediate product is produced in the first reactor and is subsequently
separated and sent to storage. The remainder of the reaction mixture is sent to a second stage reactor in
which product is formed. This product is subsequently separated and sent to storage, and unused reactant
is also separated and recycled to the front end of the process. Based upon the reason for including the

unit, each unit operation found on a PFD can be placed into one of these blocks. Although each process
may not include all the blocks, all processes will have some of these blocks.

I n Example 2.6, at the end of this chapter, different configurations will be investigated for a given

process. It will be seen that these configurations are most conveniently represented using the building
blocks of the generic block flow diagram.

2.3.4 Other Considerations for the Input/Output Structure of the Process Flowsheet

The effects of feed impurities and additional flows that are required to carry out specific unit operations

may have a significant impact on the structure of the PFD. These issues are covered in the following
section.

Feed Purity and Trace Components. In general, the feed streams entering a process do not contain pure
chemicals. The option always exists to purify further the feed to the process. The question of whether this
purification step should be performed can be answered only by a detailed economic analysis. However,