Page 420 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

use.

13.2.7 Selection of Convergence Criteria and Running a Simulation

For equipment requiring iterative solutions, there will be user-selectable convergence and tolerance
criteria in the equipment module. There will also be convergence criteria for the whole flowsheet
simulation, which can be adjusted by the user.

The two most important criteria are number of iterations and tolerance. These criteria will often have
default values set in the simulator. Unless specific problems arise, these default values should be used in
your simulations.

If the simulation has not converged, the results do not represent a valid solution and should not
be used.

When convergence is not achieved, three common causes are as follows.
1. The problem has been ill posed. This normally means that an equipment specification has been
given incorrectly. For example, see the first specification in Example 13.2 for the rigorous
column module.
2. The tolerance for the solution has been set too tightly, and convergence cannot be obtained to the
desired accuracy no matter how many solution iterations are performed.
3. The number of iterations is not sufficient for convergence. This occurs most often when the
flowsheet has many recycle streams. Rerunning the flowsheet simulation with the results from
the preceding run may give a converged solution. If convergence is still not obtained, then one
way to address this problem is to remove as many recycle streams as possible. The simulation
is then run, and the recycle streams are added back, one by one, using the results from the
preceding simulation as the starting point for the new one. This method is discussed in more
detail in Section 13.3.

Of the three reasons, the first one is by far the most common.

The most common reason for the failure of a simulation to converge is the use of incorrect or
impossible equipment specifications.

13.3 Handling Recycle Streams

Recycle streams are very important and common in process flowsheets. Computationally, they can be
difficult to handle and are often the cause for unconverged flowsheet simulations. There are ways in
which the problems caused by recycle streams can be minimized. When a flowsheet is simulated for the
first time, it is wise to consider carefully any simplifications that may help the convergence of the
simulation. Consider the simulation of the DME flowsheet illustrated in Figure B.1.1, Appendix B. This
flowsheet is shown schematically in Figure 13.5(a). The DME process is simple, no by-products are