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Inside VMG | March 2014

Tips & Tricks: Product Distribution Analysis in Distillation Curve Unit Operation

By Herbert Loria

Introduction

The composition and flow of defined distillation cuts can be tracked down through a flowsheet using the Product Distribution analysis. This is a new feature that has been added to the distillation curve unit operation in VMGSim. The Product Distribution has 7 default cuts that can be customized with different names and end temperatures.

Product Distribution

The Product Distribution can be selected in the Active Curves frame from the DistCurve Unit Operation. Once this option is selected two tabs become available: Product Distribution, which shows the composition of different cuts in the selected basis and Settings, which allows the user modifying the number, name and end temperature of the cuts for the Product Distribution.

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Product Distribution Tab

The Product Distribution tab shows the composition and flow of the different available cuts and their end temperatures. The compositions and flows are shown in the basis selected in the Summary tab or in the Configuration Frame.

The composition is based on the TBP curve and computed by the volume, mass or mole increment percent between the initial and final temperature of the defined cut. For the first cut the initial temperature is the Initial Boiling Point of the TBP curve and, the end temperature of the last cut is the corresponding to 100% distillation of the TBP curve. The flows are based on the flow from the inlet port; if the flow is not specified then the flows of the product distribution are not calculated.

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Settings Tab

In this tab the user can modify the number, name and end temperature of the Product Distribution cuts.

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Settings

The Number of Products or cuts can be set here. At least 2 cuts must be defined in order to solve the Product Distribution.

Product Distribution

The Products (cuts) name and end temperature can be modified here. For the first cut the initial temperature is the Initial Boiling Point (IBP, 0% distillation) of the TBP curve and, the end temperature of the last cut is the End Boiling Point (EBP, 100% distillation) of the TBP curve. If not known, the end temperature of the last cut is specified as the one from the previous cut; however, this end temperature will be modified to the EBP during the calculations.

It is important to note that the cut names and end temperatures defined in this tab are global to the thermodynamic case, meaning that the same cuts are used elsewhere in a flowsheet.

The default cuts are:

Lights: IBP – 31.1 °F

Straight Run Naphtha: 31.1 – 200 °F

Naphtha: 200 – 380 °F

Kerosene: 380 -545 °F

Diesel: 545 – 670 °F

GasOil: 670 – 950 °F

Resid: 950 °F – EBP

Example

In the following example, the product distribution will be calculated for a typical hydrocarbon stream. Start a new VMGSim case and add the Advanced_Peng_Robinson property package with the following components: Nitrogen, Carbon Dioxide, Methane, Ethane, Propane, iso-Butane, n-Butane, iso-Pentane, n-Pentane and n-Hexane. Set the unit set to Field and create a material streams, as shown below.

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Create a DistCurve unit operation and connect the material stream as shown below.

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Go to the Product Distribution tab and note that the distribution was only calculated for two points because the distillation curve does not cover all the default cuts.

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Go to the Settings tab, modify the cut settings in order to see more cuts in the product distribution and add the following products.

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Return to the Product Distribution tab to see a more detailed distribution based on the previously supplied cuts.

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The Product Distribution compositions and flows can be tracked down through the entire flowsheet and can be added as Independent Variables for Case Studies or Model Regressions. The number of cuts can be further increased or decreased as desired but remember that these cuts are global to the thermodynamic case and will be modified elsewhere in the flowsheet.

Software

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