Association term

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The association term is used in PC-SAFT and co-polymer PC-SAFT. It allows the model to handle more complex mixtures since it will take cross association into account.

 

3 types of sites are defined:

•Negative - Associate with positive and neutral sites

•Positive - Associate with negative and neutral sites

•Neutral - Associate with all sites

 

A complete list of schemes are implemented into VLXE

NR

Scheme

Description

1

1A

1 neutral site

2

1C

1 negative site  (active only when mixed with a molecule with at least one positive or neutral site)

3

1D

1 positive site (active only when mixed with a molecule with at least one negative or neutral site)

4

2A

2 neutral site

5

2B

1 negative site and 1 positive site

6

2C

2 negative sites  (active only when mixed with a molecule with at least one positive or neutral  site)

7

2D

2 positive sites (active only when mixed with a molecule with at least one negative or neutral site)

8

3A

3 neutral sites

9

3B

2 positive sites and 1 negative site

10

4A

4 neutral sites

11

4B

3 positive sites and one negative site

12

4C

2 positive sites and 2 negative sites

 

 

 

 

13

 

 

 

 

User(x,y,z)

Custom scheme where the caller defines the number of sites:

x: Number of negative sites

y: Number of neutral sites

z: Number of positive sites

Examples:

2B is set using: User(1,0,1)

4A is set using: User(0,4,0)

4C is set using: User(2,0,2)

 

The user can combine the schemes in any way possible.

 

The schemes 1C, 1D, 2C and 2D are for components that does not self-associate but is able to associate with other components like water or methanol.

The most common case is polar components like acetone.

However this pose the problem of obtaining the parameters needed for e.g. acetone since it cannot be determined by fitting to pure component data.

To solve this problem the "Consortium of complex fluid" has suggested that the kappa parameter is taken from the other component while the epsilon is set to zero.

The standard mixing rules can then be used.

This method may not always give the best result and VLXE therefore support user given parameters where no mixing rules are used.

 

The user therefore now has to select either automatic or user.

If "Automatic" is selected the standard mixing rules are used and the user has to give kappaAB and epsilonAB for each component.

If "User" is selected no mixing rules are used and the user has to give both the kappaAB and epsilonAB matrix.

 

For more information please see the reference: 1