Problem Recycle Convergence and Acceleration Options

(Input menu)

This window is used to specify the default convergence, tolerance and acceleration options for all the recycle loops in the current simulation. Individual recycle loops can override this default using the Individual Recycle Loop Data Window.

Usage

Select Input/Recycle Convergence command or push the $image\RECYCBUT.gif$ button on the toolbar to display this window.

Initially, all the entries in this window are filled with default values that may be overridden as desired.

Recycle Convergence Options

You can either set all the streams in the recycle loop to meet the convergence criteria, or only the recycle tear streams to meet the criteria in order to reach a solution.

You can change the iteration interval between which the recycle stream compositions should be printed and also modify the maximum number of trials (iterations) that can be performed for every recycle loop. If the loop does not converge within this maximum number of trials, a warning message will be displayed in the output report.

Recycle Stream Tolerances

These tolerances are used for converging stream properties. Large tolerance values allow faster convergence but the accuracy might be low, whereas tighter convergence tolerances may result in large calculation times. Note that when solving nested loops, the tolerance for the inside loop must be smaller than the tolerance for the outside loop.

You can select tolerance values for component, temperature and pressure parameters. The tolerances might be specified in either Absolute or Relative basis. PRO/II excludes components below a certain threshold concentration from the convergence checks. You can change the default threshold value to a new value if desired.

Acceleration Options

You can optionally choose to accelerate the recycle convergence, especially for those loops which converge asymptotically, using Wegstein or Broyden Acceleration Methods. You can select the iteration after which the acceleration should be applied. If you choose Wegstein, you may also select the number of iterations between application of the recycle acceleration techniques, and the acceleration factor's upper and lower limits.

If an acceleration option is selected, PRO/II accelerates all the tear streams by default. You may override this and specify only a selected set of streams to be accelerated. This can be done through the Accelerated Streams Window which is displayed when the Accelerated Streams button is pushed.

User Defined Recycle Loops

You may change the recycle sequence, and override the default tolerance and acceleration options specified in this window for every recycle loop in the flowsheet using the User Specified Recycle Loops Window, which is displayed when this button is pushed. This button is not available if you have selected SIMSCI method for calculation sequencing since this method automatically determines the appropriate recycle sequence. Note that if you desire to enter your own loop data, you need to specify them for all the recycle loops.

The tolerance default changes between PRO/II v9.4 and PRO/II v9.3 are summarized as below.

Recycle Tolerances	PRO/II v9.4	PRO/II v9.3
Component (Relative)	0.001	0.01
Temperature (Absolute, ⁰F)	0.01	1
Pressure (Relative)	0.01	0.01
Exclude Components with mole fraction	0.001	0.01
Number of Trials for each Recycle Loop	50	20

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