Page 46 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

Example 1.4

We locate Stream 1 in Figure 1.5 and note that immediately following the stream identification diamond a
staff is affixed. This staff carries three flags containing the following stream data:
1. Temperature of 25°C
2. Pressure of 1.9 bar
3
3. Mass flow rate of 10.0 × 10 kg/h

The units for each process variable are indicated in the key provided at the left-hand side of Figure 1.5.

With the addition of the process control loops and the information flags, the PFD starts to become
cluttered. Therefore, in order to preserve clarity, it is necessary to limit what data are presented with
these information flags. Fortunately, flags on a PFD are easy to add, remove, and change, and even
temporary flags may be provided from time to time.

The information provided on the flags is also included in the flow summary table. However, often it is far
more convenient when analyzing the PFD to have certain data directly on the diagram.

Not all process information is of equal importance. General guidelines for what data should be included
in information flags on the PFD are difficult to define. However, at a minimum, information critical to the
safety and operation of the plant should be given. This includes temperatures and pressures associated
with the reactor, flowrates of feed and product streams, and stream pressures and temperatures that are
substantially higher than the rest of the process. Additional needs are process specific. Examples 1.5–1.7
illustrate where and why information should be included directly on a PFD.

Example 1.5

Acrylic acid is temperature sensitive and polymerizes at 90°C when present in high concentration. It is
separated by distillation and leaves from the bottom of the tower. In this case, a temperature and pressure
flag would be provided for the stream leaving the reboiler.

Example 1.6

In the benzene process, the feed to the reactor is substantially hotter than the rest of the process and is