Flux enhancement during Dean vortex tubular membrane nanofiltration: 13. Effects of concentration and solute type

Abstract

Controlled centrifugal instabilities (called Dean vortices) resulting from sufficient flow in composite polyamide–poly(ether sulfone) helical membrane tubes have been used to reduce concentration polarization during nanofiltration. These vortices enhance back-migration through convective flow away from the membrane–solution interface and increased shear at the membrane–solution interface and allow for increased membrane permeation rates. As a result, solute concentrations at the membrane–solution interface and resulting osmotic-driven back flow are reduced. The performance of two sets of modules (designated Set II and Set III), each set containing a prototype vortex generating helical tubular nanofiltration (NF) element and a conventional linear element was evaluated. Nanofiltration of aqueous solutions of inorganic salts (including KCl, K 2SO 4 and K 3PO 4) and amino acids of similar molecular weight (including glutamic acid, glutamine and lysine) was performed with Set II. These experiments, designed to evaluate the effects of solute type, were conducted at the same energy consumption and transmembrane pressures. Both membrane swelling and charge effects were evident as a function of varying the pH during membrane filtration of both inorganic salts and amino acids. Both flux and rejection were higher for the helical module than the linear module during amino acid nanofiltration. A new modified phenomenological model was shown to be effective for predictive purposes for cases of responsive concentration polarization. Its applicability is validated by performing nanofiltration of aqueous MgSO 4 solutions with a new set of modules designated as Set III. Modules of Set III contained dissimilar helical and linear elements. The model was then tested against the results obtained previously.