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Aspen Physical Property System - Physical Property Models

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Zm RA<br />

Zi *,RA<br />

Vcm<br />

2 Thermodynamic <strong>Property</strong> <strong>Models</strong> 209<br />

=<br />

=<br />

=<br />

Tr = T / Tc<br />

Note: Reduced temperature Tr is always calculated using absolute<br />

temperature units.<br />

Note: Above Tr=0.99 an extrapolation method is used to smooth the<br />

transition to constant molar volume equal to the critical volume.<br />

Parameter<br />

Name/Element<br />

Symbol Default MDS Lower<br />

Limit<br />

Upper<br />

Limit<br />

MRKZRA/1 a i RKTZRA x 0.1 0.5 —<br />

MRKZRA/2 b i 0 x — — —<br />

MRKZRA/3 c i 0 x — — —<br />

MRKKIJ/1 A ij x — — —<br />

MRKKIJ/2 B ij 0 x — — —<br />

MRKKIJ/3 C ij 0 x — — —<br />

References<br />

H.G. Rackett, J.Chem, Eng. Data., Vol. 15, (1970), p. 514.<br />

C.F. Spencer and R.P. Danner, J. Chem. Eng. Data, Vol. 17, (1972), p. 236.<br />

Rackett Extrapolation Method<br />

Units<br />

The Rackett equation has a formula involving an exponent of 1+(1-Tr ) 2/7<br />

which is invalid above the critical temperature. As a result, a special<br />

extrapolation method is required for this equation. This method involves the<br />

calculation of an intermediate temperature T00 near the critical temperature.<br />

When the temperature exceeds T00, the volume is constant at the critical<br />

volume. When the temperature is between 0.99Tc and T00, a circle equation is<br />

used to smoothly interpolate the volume between the value and slope at<br />

0.99Tc and the constant value at T00.

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