Column Initial Estimates

All column algorithms are iterative in nature, starting from an initially estimated solution profile consisting of tray temperatures and flows. The initial estimate may be produced internally using an initial estimate generation method and/or provided by the user as initial profiles. Initial profiles may be used to replace selected values predicted by the estimate generators.

Unhiding the Default Reflux Estimate

If a condenser is present, or a duty on tray one, instead of using "Default" in the drop down list box, the Reflux Estimate, PRO/II starts up with the choice of "Reflux Estimate" set to Bulk Liquid/Distillate (L/D) Ratio. The associated field that becomes visible with that selection is set to 3.0.

Defaulting IEG Method to Conventional under certain Conditions

If a column is being set up, or when it’s imported from keywords, the method is set to "Conventional" if and only if both a condenser and a reboiler are present (detected by a duty on tray 1and tray N).

If it’s through keywords, the user selection on the ESTIMATE statement takes priority.

If there is no duty on either tray 1 or tray N or both, then the "Simple" choice is selected.

Transferring Default or User Supplied Reflux Estimate to a Column Specification

When the column is being set up through the GUI, there are three points where the program can intercede to enter the specification into the spec system.

From the Initial Estimates Window if the following conditions are true:

1. The column with a condenser is initialized

2. There are no reflux or vapor/liquid traffic specifications or the reflux specification in place originated from a reflux estimate in the current column WINDOW session

3. At least one totally unfilled specification is available

4. Initial Estimates window is clicked for the first time or changes are made to the reflux estimate in the Initial Estimates

Then on leaving the Initial Estimates WINDOW an MsgBox is thrown up with the following options:

Accepting "Yes" transfers the reflux to the Specification system. "No" suppresses transferring the reflux to the Specification system, and "Cancel" will return the user to the Initial Estimates WINDOW.

If a non-default value is given for "Reflux Estimate", then that value is used and the Net Liquid Rate is ignored. If the Default selection is made for "Reflux Estimate" then the value given for tray 1 Net Liquid Rate is taken. If the Default selection is made and there is not value given for tray 1 net Liquid Rate, then the default reflux ratio of 3.0 is taken as the spec.

By default, on choosing reflux ratio of 3.0 results in the following Specification/Variable WINDOW:

Upon Entering the Specification and Variables Window

If the following conditions are true:

1. The column with a condenser is initialized

2. There are no reflux or vapor/liquid traffic specifications

3. There is at least one totally unfilled specification available.

4. The user has not OKed an Initial Estimates WINDOW.

5. The Specifications and Variables WINDOW is clicked for the first time

Then, a Message Box pops up with the following option.

Since the Initial Estimates WINDOW has not been visited yet, the reflux estimate will be set to the default (normally reflux ratio of 3) and the user has not yet had the opportunity to accept this suggestion. On "Yes" the reflux ratio default will be accepted as a specification. On "No", the Specification and Variables WINDOW is loaded, but the specification that would have contained the reflux spec is left in its completely unsatisfied (red) state. On "Cancel", PRO/II returns control to the Main Column WINDOW.

Clicking OK in the main column WINDOW without entering the Initial Estimates or Specifications WINDOW

This is treated exactly as the previous case except that a "Yes" or "No" will return control to the PFD level rather than the Specification/Variable WINDOW. Cancel returns control to the Main Column WINDOW as before.

Usage

From the Column Main Window, push the Initial Estimate button.

When using an Initial Estimate Generation method:

Select the initial estimate generation "Method" from the drop down list box. See Initial Estimate Generation Methods for description of the methods.

If you have selected the Electrolytic algorithm, you may specify the number of flash iterations allowed to reach the initial composition.

Optionally, provide estimated temperatures for the Condenser, Top tray, Bottom tray, and/or Reboiler in the data entry fields provided (where applicable). See Initial Temperature Estimates for additional information.

If a condenser is present, an estimate of the column reflux may be provided by checking the desired "Reflux Estimate" option and then entering the estimate in the appropriate data entry field. See Reflux Estimate for further details.

After solution of a column, the profiles from the converged solution will normally be used to speed future calculations unless the data are resaved by pressing the OK button on the Column Main Window. Uncheck the Use previous solution for initial estimate entry to have PRO/II use your specified product stream rate estimates to calculate the initial temperature and vapor profiles for the column. See Override Calculated Product Rate Estimates for further details.

Estimated quantities may be provided in different dimensional units from those displayed by using the Unit-Of-Measure Selection Window which is displayed when the UOM button is pushed.

When Providing Initial Profiles:

When the default Initial Estimate Generation Methods data are to be overridden with user supplied values, the minimum data which must be provided for an initial estimate are Tray Temperatures and Tray Net Flow Rates for one or more phases. Temperatures must be provided for the top and bottom trays. A sufficient number of tray flow rates must be given to enable a material balance to be estimated for all of the trays. Composition estimates are sometimes helpful for highly non-ideal systems and three phase distillation calculations. Composition estimates are not needed for most problems.

For VLLE systems, net liquid rate and composition profiles may be provided for the bulk liquid and/or for the individual liquid phases.

Push the appropriate buttons to begin entry of Initial Profiles data.

When the Initial Estimate Generation facility is used, the Initial Profiles feature may be used to replace selected values generated by the Estimate Generator.

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