Alpha Selection Window

The pure component alpha formulations are used for methods based on the cubic Equation of State. The alpha function controls pure component properties (vapor pressure, enthalpies and entropy), as opposed to the interaction parameter, which controls the mixture properties. The purpose of this window is to select the source of the alphas used by an equation of state.

Usage

From the Thermodynamic System - Modification Window, click the Enter Data button for the property method of interest.
From the Thermodynamic Property Modification Window, click the Enter Data button corresponding to Data Item "Alpha Selection".

The K-value (VLE) and (LLE) properties immediately enable the Alpha Selection button. SLE allows entering solubility of solid in the liquid instead of alpha or acentric factor data. Other properties (e.g., Liquid Enthalpy, Density, Entropy, Vapor Enthalpy, Density, Entropy) require checking the Enter Data Specifically... check box to enable the Alpha Selection button.

The following Alpha Selection options designate sources of data that supply equation of state alphas for all components generally; but there is no guarantee data is complete for all components.

Acentric Factor Formulation Selects the Acentric Factors option in place of other alpha forms. This is the default selection for the SRK and PR methods. Using this option without supplying any coefficients causes PRO/II to compute the alpha value as a function of acentric factor and reduced temperature {alpha = f(Tr, acentric factor)}. Alternatively, the user may choose the Acentric equation form and enter a value for the alpha value as Coefficient 1. In this case, Coefficient 1 typically is used directly as the alpha term and coefficients 2 and 3 are ignored.

SimSci Alpha Databank This option fills component alpha values from the SIMSCI databank (and possibly others). This is the default selection for all Defined Systems except the SRK and PR methods.

Equations of state typically include a component-specific "Ai" term such that:

Ai = alphai * Aicritical where alphai = f( Tr, C1, C2, and C3)

In this generalized form, Aicritical is a constant, and the alpha value is a function of reduced temperature (Tr), C1, C2, and C3. Note that alpha data from a library may appear using any of the available equation forms. Data for each component may use a different equation form. The SimSci option neither requires nor implies a preference for one equation form over any other.

Users may override data from the alpha data banks or supply their own correlations for alpha by selecting an equation form and supplying up to 3 coefficients. Supplying coefficients is most typical when library data for a component alpha is missing. Properties taken from password protected libraries are shown with text ”PROT”.

To supply alpha coefficient values, proceed as follows.

Alpha Coefficients for Components Press this button to open the Alpha Coefficients Data Entry Window. Refer to the screen image below. The window displays a grid that includes every component in the simulation. Each component occupies one row of the grid.

Repeat the following for each component of interest.

Select a component.
Choose an equation from the Alpha Form drop-down list box (to the right and on the same row) for the component. Alpha equation forms are defined in Table 4.6 of the Thermodynamic Data Keyword Manual.
Enter equation coefficient values in the Coefficient fields on the same row of the grid.

User-supplied Alpha or Acentric factor data

When data entry is complete, press the OK button to save the data and exit the window. Pressing Cancel discards any data that was entered.

Note: Alpha databanks are also available for BioLib, Dippr libraries (Lite/Full and Sponsor), and Process databanks. Select databank to match the appropriate Pure component databank. The new SIMSCI is the default, if user wants compatibility with previous version, user needs to insert PROCESS as the first in the order.

Pure component databanks and corresponding Alpha databanks are listed below:

Library Name	“Pure Component” Databank	Recommended “ALPHA” Databank	Description
Edlib	SIMSCI	SIMSCI (Note 1)	This bank contains ALPHA parameters for SIMSCI bank.
Edlib	PROCESS	PROCESS	This bank contains ALPHA parameters for PROCESS bank. Important: This bank was previously named “SIMSCI.”
BioLib	BIOETHNL	BIOETHNL	This bank contains ALPHA parameters for BIOETHNL bank.
BioLib	BIOFUELS	BIOFUELS	This bank contains ALPHA parameters for BIOFUELS bank.
Dippr	DIPPR	DIPPR (Note 1)	This bank contains ALPHA parameters for DIPPR bank.

Note1: The SIMSCI and DIPPR Alpha banks actually provide two sets of alpha parameters:

The original parameters for supercritical components (Tc < 273.15 K). These parameters also appear in the PROCESS Alpha bank.
New parameters for non-supercritical components

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