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Abstract

Using 0.1 molar brine solutions of NaCl, Na₂SO₄, MgSO₄, NaNO₃, CaCl₂, Na₃PO₄, and AlCl₃, the transport parameters of the phenomenological relationships such as the the hydrodynamic, ℓ_p, and the osmotic, ℓ_π, permeability as well as the asymptotic salt rejection, r_∞, were determined for asymmetric Du Pont Aramid (B-9) flat sheet membranes by means of hyperfiltration experiments. Moreover, the same transport parameters were obtained for hollow-fine fiber modules made from the same polymer (B-9) as well as the polymer employed in seawater devices (B-10) using 0.1 molar and 0.5 molar NaCl brine solutions. In addition, the transport parameter ℓ_p, and the reflection coefficient σ were measured in dialysis-osmolis experiments as functions of NaCl concentration using flat sheet membranes (B-9) and applying different boundary conditions.

The asymptotic salt rejection, r_∞, is always beyond 0.9800 for all the salts used. Depending on the membrane sample, even values beyond 0.9900 are obtained for the different salts. The experimental findings clearly demonstrate that there is no qualitative nor quantitative difference between flat sheet membranes and hollow-fine fibers if the comparison is made on the basis of those transport parameters which do not depend on the thickness of the active layer such as the salt permeability, P_s, for instance. The hydrodynamic permeability of the polyamide membranes used varies drastically with the salt applied due to swelling effects. The lowest hydrodynamic permeability results for Na₃PO₄ while the largest value is obtained with NaCl brine solutions. Furthermore, the flat sheet membranes exhibit strong asymmetry effects in dialysis-osmosis experiments under boundary conditions far from near-equilibrium. These asymmetry effects originate from concentration polarization effects in the porous matrix of the asymmetric membranes.