Page 409 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

The solution sequence for flowsheets containing recycle streams is more complicated, as shown in Figure
13.3. Figure 13.3(a) shows that the first equipment in the recycle loop (C) has an unknown feed stream
(r). Thus, before Equipment C can be solved, some estimate of Stream r must be made. This leads to the
concept of tear streams. A tear stream, as the name suggests, is a stream that is torn or broken. If the
flowsheet in Figure 13.3(b) is considered, with the recycle stream torn, it can be seen, provided
information is supplied about Stream r2, the input to Equipment C, that the flowsheet can be solved all the
way around to Stream r1 using the sequential modular algorithm. Then compare Streams r1 and r2. If they
agree within some specified tolerance, then there is a converged solution. If they do not agree, then
Stream r2 is modified and the process simulation is repeated until convergence is obtained. The splitting
or tearing of recycle streams allows the sequential modular technique to handle recycles. The
convergence criterion and the method by which Stream r2 is modified can be varied, and multivariable
successive substitution, Wegstein, and Newton-Raphson techniques [2,3] are all commonly used for the
recycle loop convergence. Usually, the simulator will identify the recycle loops and automatically pick
streams to tear and a method of convergence. The tearing of streams and method of convergence can also
be controlled by the user, but this is not recommended for the novice. Note that heat integration (Chapter

15) introduces recycle streams.

Figure 13.3 The Use of Tear Streams to Solve Problems with Recycles Using the Sequential Modular
Algorithm

13.2 Information Required to Complete a Process Simulation: Input Data

Referring back to Figure 13.1, each input block is considered separately. The input data for the blocks
without asterisks (1, 3, 4, and 6) are quite straightforward and require little explanation. The remaining
blocks (2, 5, and 7) are often the source of problems, and these are treated in more detail.