Page 153 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

5.3 Recycle and Bypass Streams

It is important to be able to recognize recycle and bypass streams in chemical processes. When
identifying recycle and bypass streams, we look for flow loops in the PFD. Any time we can identify a
flow loop, we have either a recycle or a bypass stream. The direction of the streams, as indicated by the
direction of the arrow heads, determines whether the loop contains a recycle or a bypass. The following
tactics are applied to flow loops:
Tactic 4: If the streams in a loop flow so that the flow path forms a complete circuit back to the point
of origin, then it is a recycle loop.
Tactic 5: If the streams in a loop flow so that the flow path does not form a complete circuit back to
the place of origin, then it is a bypass stream.

It is worth noting that certain pieces of equipment normally contain recycle streams. In particular,
distillation columns very often have top and bottoms product reflux streams, which are essentially recycle
loops. When identifying recycle loops, we can easily determine which loops contain reflux streams and
which do not. Example 5.3 illustrates the procedure for identifying recycle and bypass streams in the

toluene hydrodealkylation PFD.

Example 5.3

For the toluene hydrodealkylation PFD given in Figure E5.1, identify all recycle and bypass streams.

The recycle loops are identified in Figures E5.3(a) and E5.3(b). The main toluene recycle loop is
highlighted in Figure E5.3(a), and the hydrogen recycle loops are shown in Figure E5.3(b)(a) and E5.3(b)
(b). There are two reflux loops associated with T-101, and these are shown in Figures E5.3(b)(c) and
E5.3(b)(d). Finally, there is a second toluene recycle loop identified in Figure E5.3(b)(e). This recycle
loop is used for control purposes (see Chapter 21) and is not discussed further here. The logic used to
deduce what chemical is being recycled in each loop is discussed in the next example.