Tower fitting

The Tower Fitting dialog allows you to calculate a flowsheet's fractionation parameters by fitting distillation data for crude products. Before you can use the tower fitting dialog you must define a flowsheet's topology using the Flowsheet Manager.

Define the flowsheet topology

Use the Flowsheet Manager or Graphical Flowsheet editor to create the flowsheet topology ("Create flowsheet topologies" on page 56).

Enter test run data for the refinery streams

Click the Fit Tower button to open the Tower Fitting dialog.

The Tower Fitting dialog allows you to specify the nature of the separation into products. This can be done in one of two ways:

• By entering parameters to define the separation efficiency (cut points and fractionation indices), or

• By entering measured distillation data ("test run data") for each product, and allowing the tower to be configured by fitting to this data.

Entering tower parameters

The separation efficiency for a tower can be entered directly in the Flowsheet Manager, or in the Flowsheet Parameters grid. For each stage of the separation you must enter the following information:

• Product Name

• ECP

• Stripping FI

• Rectifying FI

When using the ECP/SI separation model, Stripping FI and Rectifying SI are replaced by Bottom SI and Top SI, respectively.

For a discussion of the meaning of these terms see Tower models.

After entering any flowsheet parameters to prevent Flowsheet fitting changing the entry click the Lock Parameter icon . The icon will change to a locked padlock to indicate the entry cannot be changed during Tower Fitting. The value can still be manually edited by the user however. To allow Flowsheet Fitting to change the parameter click the icon again to unlock the entry and refit the tower data.

Test runs are often performed on distillation units, and the results will often consist of yield measurements for each product, and distillation profiles for each product. To enter test run data of this sort the typical workflow is to:

• Enter (or load) the test run data

• Save it to a file (if required)

• Fit the data

• Review the fit

Entering test run data

You can enter a variety of test run data in Process Model Manager Reference Manual, including:

• Yields: In either weight or volume units

• Distillation Profiles: In any of true yield (by weight or volume), D86 yield, D1160 yield, or SIMDIS yield.

To configure your data, you should enter:

• Names for all products. It is by these names that the products are identified when they need to be accessed.

• Yields in either weight or volume units for the products. The products are in the order residue to gas. You may only omit the gas yield, which will be filled in to make the sum equal to 100% yield. All other yields must be provided.

• Distillation Profiles in any of true yield (by weight or volume), D86 yield, D1160 yield, or SIMDIS yield. Changing the distillation percentage values and distillation method is achieved by double-clicking the corresponding heading. This launches editable, linked property controls. Note that you may have several distillation methods for one tower, or even for one product. You may omit distillation profiles for any product; however, some distillation data must be supplied for at least one product.

Test run data is entered in the Measurement Grid of the Tower Fitting dialog. Select the test method used to obtain the test run data by selecting the method from the list of options shown by double-clicking the test-method heading in each column. Use the left-hand column to enter the percentage points for the test run data i.e. the percentage of fluid distilled at the appropriate measurement temperature.

Tip: It is not necessary to include yields for all streams. Where yields are unknown they will be calculated during the fitting process. For example if the overhead yield is not known exactly this may be left blank during data entry and during fitting the overhead yield will be calculated.

Warning: Yields for each tower are not normalised and expected to sum to 100%. As such the percentage yield for each product should be relative to the input feed to the tower. For an atmospheric tower this would be the yield relative to whole crude, but for a vacuum tower the yield of each stream would be relative to the yield of the tower input (typically atmospheric residue).

Yields must be entered as a percentage. It is not possible to use other units of measure such as ratio.

Loading and Saving

It is possible to load and save the data you have entered for each tower as a Tower Fitting Data file (*.tfd ) by using the Load and Save buttons.

The test run data you enter for each flowsheet is saved along with the flowsheet parameters in Process Model Manager Reference Manual. When you export the flowsheet the test run data is exported along with the parameters in the file.

Fit the test run data a review the results

When data has been correctly entered for each product, pressing Fit will fit the entered data to provide the best-fit set of parameters consistent with the measured distillation and yield data, according to the separation model you have chosen ("Tower Models" on page 58).

After fitting the test run data, you may review:

• The numerical quality of the fit

• The generated parameters for the fit, and

• The graphical quality of the fit.

The numerical quality of the fit may be reviewed by checking the values in the upper right of the grid cells. These values are the fitted values, and allow you to review how closely the yields and distillation profiles were matched during the fitting process.

The generated parameters for your fit are shown in the grid. Any that you wish to modify from the fitted values may be changed in this grid.

Several buttons are available for working with the Tower Fitting data and results:

• Fit - calculates the system fractionation parameters based on the entered test run data

• Clear - deletes any distillation present in the Distillation Grid but does not affect any entered values for ECP or FI

• Import - imports tower fitting data from an external .tfd file

• Export - exports tower fitting data to a .tfd file

• Show/Hide Graph - shows or hides the Distillation Graph.

Tower Fitting data can be exported to an external .tfd file, and these files can be imported to the Tower Fitting dialog. Tower fitting data can only be imported to the Tower Fitting dialog if the flowsheet has the same number of flowsheet streams. Tower fitting data is saved in a flowsheet file and will be exported if the flowsheet is exported.