HYSYS Operations Guide

Logical Operations 5-205
of the material entering the flowsheet to leave. An
example of this occurs in gas plants when you are trying
to make a liquid product with a low vapour pressure and
a vapour product which must remain free of liquids even
at cold temperatures. Often, this leaves no place for
intermediate components like propane and butane to go,
so they accumulate in the plant recycle streams. It is
also possible that the tolerance is too tight for one or
more of the Recycle variables and cannot be satisfied.
This can readily be determined by examining the
convergence history, and comparing the unconverged
variable deviations with their tolerances.
The Monitor tab provides a history of the Recycle
calculations.
• The logical operations (such as the Recycle, Adjust and
Controller) are different from other operations in that
they actually modify the specifications of a stream. As a
result, if you remove any of these operations, the outlet
stream specifications remain. Thus, nothing in the
flowsheet is “forgotten” for these operations. You can
Delete or Ignore a Recycle when you want to make
flowsheet modifications, but do not want to invoke the
iterative routines.
• Tolerance settings are important to a successful Recycle
solution. This is especially true when multiple recycles
are involved. If there is no interaction among the
recycles, or if they are inter-connected and are being
solved simultaneously, tolerance values can be identical
for all Recycles if desired. However, if the Recycles are
nested, tolerances should be made increasingly tighter
as you go from the outermost to the innermost Recycle.
Without this precaution, the outside Recycle may not
converge.
Maximum Number of Iterations
When HYSYS has reached the maximum number of iterations, a
warning message appears stating that the Recycle failed to
converge in the specified number of iterations. You can then
choose whether or not to continue calculations.
If you are starting a new flowsheet, use a small number of
Maximum Iterations, such as 3. Once it is evident that the
calculations are proceeding well, the count can be increased.
The iterations required depend not only on the complexity of
5-205