Improving flowsheet convergence in HYSYS
Written by James Holoboff, Process Ecology

For many HYSYS flowsheet applications, logical operations such as the Adjust or Recycle are required
to ensure that convergence can be achieved. At times, these operations will not converge and
troubleshooting can be very difficult.

Basic troubleshooting

For Adjust operations, basic troubleshooting tips include:

  • Ensuring that the tolerance is not too small
  • If the Adjust is converging too slowly, increase the step size or increase the maximum number
    of iterations.
  • If the Adjust is significantly overshooting the target, decrease the step size
  • It is good practice to set minimum or maximum values, if applicable. For example, if you are
    adjusting flowrate, set the minimum to zero.
  • If the Adjust is not "moving" the adjusted variable, change the solution method from Broyden to
    Secant, or vice versa.

For Recycle operations:

  • If the Recycle converges but stream conditions or compositions for the inlet and outlet streams
    are not close enough, decrease the "Sensitivity" on the Variables page.
  • Consider increasing the number of iterations to avoid having to prompt HYSYS to continue
    solving once it has reached the maximum number of iterations.

Using a Recycle instead of an Adjust

In our experience, recycles converge more consistently and more quickly than Adjust operations. Often,
Adjust operations will simply not converge unless the solution method is toggled from Broyden to
Secant, or vice versa. However, a Recycle operation will rarely have a convergence problem if the
flowsheet is properly set up.

Consider the following case - a simple choke plant cools gas through the gas/gas exchanger to the
choke valve where pressure is reduced in order to meet dewpoint specifications. Gas from the low
temperature separator (LTS) exchanges with the choke plant inlet gas in the gas/gas exchanger. If the
gas/gas exchanger has a minimum approach specification, this requires an iterative calculation - using
an Adjust, the temperature of stream "From G/G" would be adjusted until the minimum approach spec
for the gas/gas exchanger is achieved.
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Figure 1 - Using an Adjust for convergence - temperature of "From G/G" until minimum approach of
gas/gas exchanger matches the target value

Alternatively, set up this flowsheet using a Recycle operation. Create a minimum approach specification
in the gas/gas exchanger, add the recycle operation and allow HYSYS to calculate the "From G/G"
stream temperature. In our experience, this results in a flowsheet which will solve consistently.
Figure 2 - Using a Recycle for convergence - a minimum approach specification is set up in the
gas/gas exchanger, and the temperature of "From G/G" is calculated

Using Calculation Levels

When there are multiple adjust or recycle unit operations you may need to specify which adjust
operation to converge first. This may be useful if you find that adjusts are "competing" with each other,
resulting in the flowsheet not converging (or converging very slowly). Go to Simulation-Main Properties,
and modify the Calculation Level on the "Calc Levels" tab. To force HYSYS to solve ADJ-1 before ADJ-2,
set the calculations levels to 3500 and 4000, respectively.

Checking tolerances are consistent

Often, the default tolerances in the recycle are not tight enough to ensure that the flowsheet properly
convergences. For example, if you are solving a column to a tight compositional tolerance (for example,
in a glycol or amine loop), then it is important to ensure that the recycle sensitivity for composition is
small enough.

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