Use a reference fluid

When fitting a distillation unit or a flowsheet you can use yield and property profiles from a crude, or blend of crudes, from the database. To do so, follow these steps:

1. Select the Use Reference Fluid check box in the top left corner of the Distillation Unit Fitting dialog. Note how additional rows appear under the yield row in the top grid.

2. Click the button. The Reference Fluid dialog opens.

3. Click the Add button. The Select Crudes dialog opens. Choose a crude and click OK. The crude is added to the list in the Reference Fluid dialog.

4. If needed, repeat step 3 to add more crudes.

5. If you have added more than one crude:

   • Double click cells in the Ratio column to change the ratios of each crude in the blend. The values in the % Wgt and % Vol columns are updated automatically.

   • Click the Weight Ratios or Volume Ratios option button, depending on the ratios you want to use to create your blend.

6. Review the list of crudes. If needed, click the Edit and Remove buttons to replace or delete the selected crude, respectively.

7. Click OK. The Reference Fluid dialog closes. Values for yields and distillation profiles are added to the grid, in the lower right corner of each cell.

Once the Use Reference Fluid option is enabled, you can add more properties below the yield row in the top grid. Double click on any cell in the first column to reveal a drop-down menu with a list of available properties. If you have already selected a crude or a blend, stream values for the selected properties appear in the lower right corner of the cells of the row.

In depth

When no reference yield is selected the yield curve of the mix is reconstructed from the entered yield and distillation data. When Use Reference Fluid is selected, Assay uses the yield (and, if required, property) profile of the crude or blend to assist the fitting process. See Enter test run data for the refinery streams.

Where only yield and distillation information is entered, Assay initially creates a yield curve from the entered reference blend. The fractionation parameters (ECPs and FIs) are then adjusted so that when this generated yield curve is recut, the resultant distillation profile for each product most closely matches the entered distillation data for the products (that is, the difference in calculated distillation data and entered distillation data is minimised).

Where yield and property data is entered then the initial crude blend is used to create the target yield curve and target property profiles - during fitting the fractionation parameters are adjusted so that when these target profiles are recut using the generated fractionation parameters the resultant product distillation profiles and recut property values most closely match the entered distillation and product property data.

For crude independent fitting in both cases the Reference Fluid crude blend is only used for fitting and to assist calculation of the fractionation parameters - the resultant parameters are distillation unit specific and can be re-applied to different crudes or crude mixes. For example you may have a set of data for a crude mix of 50:50 Arab Light:Nemba which shows an AGO cloud point of -15°C. This crude mix is defined as the reference yield and the product quality entered - during fitting fractionation parameters will be calculated so that when the cloud point of the Arab Light:Nemba mix is recut then the resultant AGO cloud point is close to -15°C. This set of fractionation parameters can still be applied to a different crude mix however, as the crude mix is only used to help calculate the final product property to increase fitting accuracy, and the underlying fractionation parameters are still crude independent. See Select a separation model in Create a flowsheet.

When using Reference Fluids it is important to ensure that the crude mix itself is well defined - this is especially important when using property data for tuning along with distillation data - as property data makes use of assay property profiles these must closely reflect the material in the mix in order to ensure accurate fitting. As such:

• make sure the entered Reference Fluid blend matches exactly the distillation unit blend feeding the unit during data collection - it may be appropriate to use BlendAnalysis to better calculate blend compositions. See BlendAnalysis.

• always ensure that an appropriate assay of each crude is used - the properties of the chosen assay should match closely the actual crude cargo - it may be appropriate to use AssayUpdate to generate better representations of the material actually being processed at the time data was collected. See AssayUpdate (old?).

• avoid using crude runs where the crude mix contains a large amount of poorly defined product - for example avoid runs with a significant slop or recycle percentage unless these are well defined via accurate assays

Take care if locking any fractionation parameters and using Reference Yield - the locked parameters must be compatible with the generated reference yield and property profiles.

When entering property data for the distillation unit products fitting takes into account the reproducibility of the property based on the standard test method associated with the property. Fitting ensures that differences in properties such as pour point and density with very different reproducibilities are consistently fitted.