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Stefan

Aphi in Pitzer model

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Hello

I have trouble keeping compatibility of thermodynamic data between datasets for different geochemical codes. Thus, I would like to know some details on the Pitzer formalism incorporated in GWB.

Obviously, the Aphi parameter can be individually set for a specific dataset (and for the different temperatures herein) by the debye huckel a (adh) parameter. For the thermo_hmw dataset you take adh(25°C) = 0.5092. This corresponds with a value for Aphi of 0.3909 what when rounded is the value from Bradley and Pitzer 1979 (J. Phys. Chem 83/12) for 25°C and saturated pressure. In the original HMW 84 paper 0.39 was used. Looking in Archer & Wang 1990 ( J. Phys. Chem. Ref. Data 19/2) Aphi should be 0.39148. On page 351 in Craigs “green book†a value of 0.3920 is documented for calculating the Etheta values.

Further, the values provided for Aphi in the thermo_hmw dataset agree with those of Bradley and Pitzer 1979 at 0, 25 and 60°C. At higher temperatures small deviations exist.

Theses differences in Aphi (at 25°C and other temperatures) are small and thus leading to small differences in the resulting activity coefficients. For the application on “large scale real world problems†one can neglect this. But by trying to build a database with compatible thermodynamic values this makes the difference.

Thus, I have two questions:

1) What is the value for Aphi used in GWB (or in different parts of GWB) for calculating activity coefficients following the Pitzer formalism?

2) What is the function behind the temperature dependency of Aphi in GWB datasets and - I assume - all other values listed in the same way for different temperatures?

Best regards,

Stefan

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Thus, I have two questions:

1) What is the value for Aphi used in GWB (or in different parts of GWB) for calculating activity coefficients following the Pitzer formalism?

2) What is the function behind the temperature dependency of Aphi in GWB datasets and - I assume - all other values listed in the same way for different temperatures?

Hi Stefan:

A-phi is a fitting parameter, not a fundamental constant. The value used for A-phi in the green book appendices is a value used to parameterize that method for calculating E-theta. The values in the thermo datasets correspond to the virial coefficients in those datasets, according to the source that compiled them. There's no need to worry about the fourth decimal place in a calculation that is not even accurate to two decimal places.

As far as your temperature dependency question, Aphi = 2.303 adh/3 (p. 118 of the green book). As you noted, adh values are given in the thermo dataset for the 8 temperature values defined for the database. A polynomial fit through the 8 values allows GWB to compute values at other temperatures.

I hope that helps,

Tom Meuzelaar

RockWare, Inc.

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