Carbon Number (Cn) Analysis Calculator
The objective of this communication is to present the details of the Carbon Number (Cn) Analysis Calculator unit operation. This unit operation is able to produce Cn compositional analyses, of any hydrocarbon stream, similar to those reported by analytic laboratories as well as to report the physical properties of a specific Carbon number “plus” (Cn+) fraction. The usage of this unit operation is also illustrated with the help of an example.
Cn Analysis Calculator in VMGSim
The Cn Analysis Calculator unit operation contains two sets of calculations: Cn Compositional Analysis and Cn+ Fraction Properties. The former produces a detailed compositional analysis based on single carbon numbers or carbon number groups, and the later calculates physical properties for a specified Cn+ fraction or for the bulk composition.
Cn Compositional Analysis
The detailed compositional analysis can be based on two inputs: 1) the mass, mole or volume basis composition entered in the unit operation or, 2) the TBP distribution from the composition entered in the unit operation. These options can be selected from the Calculation Mode box:
The compositional analysis is first defined by an initial and a “plus” carbon number and then it can be organized by Single Carbon Numbers (Single Cn) or Carbon Number Groups (Cn Groups).
If Single Cn is selected, the Cn Analysis will be built by individual carbon numbers meaning that the composition attached, for instance to C7, will be formed by the compositions of all hydrocarbons with 7 <= Cn < 8.
When the Cn Groups option is selected the number of carbon number groups and their corresponding initial Cn must be specified. The carbon number groups can have different sizes as shown below:
The carbon numbers can be defined not only by the carbon numbers themselves but also by boiling points and molecular weights. The boiling points and molecular weights boundaries for each carbon number group can be based on the widely accepted definition given by Katz and Firoozabadi  or they cay can be customized.
Cn+ Fraction Properties
This option provides the physical properties of a specific Cn+ fraction, the plus fraction can be defined by carbon number, boiling point or molecular weight. The plus fraction is thus defined by components with property values higher than the specified value for the plus fraction, for instance, if the plus fraction is defined by Cn = 7, then all components with Cn >= 7 will form the plus fraction; if the plus fraction is defined by boiling point = 70 C, then the plus fraction is formed by components with boiling points above 70 C.
The available plus fraction physical properties are fraction (in mass, mole or volume basis), molecular weight, density at a specified temperature and pressure, and saturation pressure at a given temperature. As complement, physical properties of the bulk mixture can also be calculated.
To start, open the Atmospheric Crude Tower.vmp case from the Flowsheeting Examples in the Documentation folder of your VMGSim installation directory: C:\Program Files (x86)\VMG\VMGSimPkg\Documentation\Manual Examples\Flowsheeting Examples\ Atmospheric Crude Tower.vmp
Add a Cn Analysis unit operation to the case by dragging the unit operation icon from the shapes window to the flowsheet as illustrated in the next figure:
Now, clone the out port of the Crude Feed stream, attach it to the Cn Analysis unit operation and open the unit operation.
The Summary tab shows the two available options in the unit operation. The Cn Composition Analysis option opens up the Cn Analysis and Cn Analysis Plot tabs.
The Cn Analysis tab contains the Cn distribution itself as well as the options available for this calculation.
If the Cn Analysis is organized by option is switched to Cn Groups, then one can enter the number of Cn groups and the initial Cn of each group.
The Cn Properties from option can be changed to User to enter customized Cn properties like boiling points.
The Cn Analysis Plot shows the distribution plot based on single carbon numbers or Cn groups.
When the Cn+ Fraction Properties option is selected, a new tab containing the Cn+ fraction properties is opened.
There are no restrictions regarding property packages to use this unit operation, also any hydrocarbon stream can be analyzed; the hydrocarbons can be defined as pure components, Oil pseudo components, PIONA Slate components or regular pseudo components.
 Katz, D.L. and Firoozabadi, A. Predicting the Behavior of Condensate/Crude Oils Systems using Methane Interaction Coefficients. Journal of Petroleum Technology 30, 11, 1649-1655, 1978
Please contact your local VMG office for more information.
Herbert Loria, PhD, P.Eng., VMG Calgary