Page 155 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

It is important to remember that flow diagrams represent the most meaningful and useful documents to
describe and understand a process. Although PFDs contain a lot of process information, it is sometimes
necessary to apply additional knowledge about a unit operation to determine which chemicals are
contained in a recycle stream. This idea is demonstrated in Example 5.4.

Example 5.4

Provide preliminary identification of the important chemical species in each of the three recycle streams
identified in Example 5.3. See Figures E5.3(a), E5.3(b)(a), and E5.3(b)(b).
Figure E5.3(a)
Stream 11: This is the bottoms product stream out of the distillation tower that provides the product
benzene as distillate. The bottoms product stream must have a lower volatility than benzene. The
only possible candidate is toluene. Stream 11 is essentially all toluene.
Figures E5.3(b)(a) and E5.3(b)(b)
Two undesignated streams leave splitter s-102: One stream leaves as part of a product stream and
joins with other streams to form Stream 16. The other stream passes through C-101 to splitter s-101.

The input and the two streams leaving s-101 have the same composition. If we know any of the
stream compositions, we know them all. In addition, methane is a reaction product and must leave
the process. There are only two streams that leave the process, namely Streams 15 and 16. Because
the methane is unlikely to be part of the benzene stream, it must therefore be in the stream identified
as fuel gas, Stream 16. The assumption is made that the product stream leaving is gaseous and not
pure methane. If it were pure, it would be labeled methane.
The only other gas that could be present is hydrogen. Therefore, the fuel gas stream is a mixture of
methane and hydrogen, and all three streams associated with s-101 have the same composition of
methane and hydrogen.
The stream that leaves splitter s-102 and goes through compressor C-101 to splitter s-101 is split
further into Streams 5 and 7. All streams have the same composition.
Stream 5 then mixes with additional hydrogen from Stream 3 in mixer m-103. The stream leaving m-
103 contains both hydrogen and methane, but with a composition of hydrogen greater than that in the