The present book contains a comparison of existing theoretical
models developed in order to describe membrane separation
processes. In general, the permeation equations resulting from
these models give inaccurate predictions of the mutual effects of
the permeants involved, due to the simplifications adopted in their
derivation. It is concluded that an optimum description of
transport phenomena in tight (diffusion-type) membranes is achieved
with the "solution-diffusion" model. According to this model each
component of a fluid mixture to be separated dissolves in the
membrane and passes through by diffusion in response to its
gradient in the chemical potential. A modified Flory-Huggins
equation has been derived to calculate the solubility of the
permeants in the membrane material. Contrary to the original
Flory-Huggins equation, the modified equation accounts for the
large effect on solubility of crystallinity and elastic strain of
the polymer chains by swelling. The equilibrium sorption of liquids
computed with this equation was found to be in good agreement with
experimental results. Also, the sorption of gases in both rubbery
and glassy polymers could be described quan titatively with the
modified Flory-Huggins equation without any need of the arbitrary
Langmuir term, as required in the conventional "dual-mode" sorption
model. Furthermore, fewer parameters are required than with the at
least identical accuracy."
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