(395ah) Simple Prediction of Adsorption Henry Constants for Supercritical n Alkanes On 5A Zeolite

By  Kevin F. Loughlin and Dana
Abouelnasr, Department of Chemical
Engineering , American University of Sharjah, PO Box 26666, University City,
Sharjah UAE. email: kloughlin@aus.edu
& dabouelnasr@aus.edu ;Tel: (971) (6) 515-2986; FAX: (971) (6) 515-2979           

ABSTRACT

A simple model is proposed to calculate
Henry constants for supercritical n alkanes on 5A zeolite using mid isotherm
data. The Gaussian isotherm contains three model parameters, q_maxthe saturation loading_, s_e
the standard deviation and P₅₀the log mean
pressure for the entire isotherm. Since the slope of the Gaussian isotherm
approaches zero as the pressure approaches zero, the equation dlnq/dlnp
= 1 is used
instead for the derivation of the Henry constants. Using this equation,
the dimensionless temperature-independent Henry constant may be derived as

where  is
expressed as K_H*P₅₀/q_max. Z_H
is the Z value for the Gaussian model at the point where dlnq/dlnp
= 1, and σ_e is the standard deviation of the isotherm. 
For supercritical n alkanes on 5A zeolite, σ_e is equal
to ln 10 and Z_H is equal to -1.92.  Upon substitution K_H^*
becomes:

The temperature-independence of  occurs
because the temperature dependence of K_H is cancelled by the
temperature dependence of P₅₀ the log mean pressure for the
entire isotherm. It is important to note that P₅₀ is
calculated at the middle of the isotherm. Therefore Henry's constants can be
estimated for isotherm data far from the Henry's Law region. Using the equation,
,
the model is observed to satisfactorily predict the Henry's law data for all
supercritical n alkane isotherms on 5A zeolite.

To completely predict Henry's constants
for this system, a predictive model is incorporated based on a correlative
approach for 63 isotherms for carbon numbers varying from 1 to 10. The
calculated values from this approach are also in remarkable agreement with
reported experimental values. A summary of the equations needed to predict the
properties of supercritical isotherms of n alkanes on 5A zeolite is provided.

KEYWORDS: separations, Henry constants, 5A
zeolite, n alkanes, supercritical isotherms