Page 156 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

other gas streams discussed.
Finally, Stream 7, which also leaves splitter s-101, flows back to the reactor and forms the third
recycle stream.

Before the analysis in Example 5.4 can be accepted, it is necessary to check out the assumption used to
develop the analysis. Up to this point, we have used the skeleton flow diagram that did not provide the
important temperatures, pressures, and flowrates that are seen in the completed PFD (Figure 1.5). Figure
1.5 gives the following information for the flowrates of reactants:
Hydrogen (Stream 3): 572 kg/h (286.0 kmol/h)
Toluene (Stream 1): 10,000 kg/h (108.7 kmol/h)

Based on the information given in Table 1.5, only 108 kmol/h of hydrogen reacts to form benzene, and
178 kmol/h is excess reactant that leaves in the fuel gas. The fuel gas content is about 40 mole % methane
and 60 mole % hydrogen. This confirms the assumption made in Example 5.4.

5.4 Tracing Nonreacting Chemicals

Chemical processes often contain nonreacting, or inert, compounds. These chemicals must appear in both
the input and output streams and are neither created nor destroyed in the process. Unlike the reactants, it
makes no difference in what direction we choose to trace these nonreacting chemicals. You can trace in
the forward direction, the backward direction, or start in the middle and trace in both directions. Other

than this additional flexibility, the tactics provided above can be applied to all nonreacting chemicals.

5.5 Limitations

When the tracing procedure resorts to combining several unit operations into a single system that provides
a single stream, the path is incomplete. This can be seen in the paths of both product streams, methane and
benzene, in Figure E5.1.
Benzene: The benzene flows into and out of the distillation system as the figure shows. There is no
indication how it moves through the internal units consisting of V-104, s-103, E-104, E-106, and T-
101.
Methane: The methane flows into and out of a system composed of V-102, V-103, s-102, and m-
104. Again, there is no indication of the methane path.

In order to determine the performance and the flows through these compound systems, you need more
information than provided in the skeleton PFD, and you must know the function of each of the units.

The development given in the previous sections used only the information provided on the skeleton PFD,
without the description of the unit operation, and did not include the important flows, temperatures, and
pressures that were given in the full PFD (Figure 1.5) and the flow table (Table 1.5). With this additional
information and knowledge of the unit operations, you will be able to fill in some of the paths that are yet
unknown.

Each step in tracing the flow paths increases our understanding of the process for the production of