(484d) An Electrokinetic Model for Combined Fouling During Cross Flow Filtration of Electrolyte Solution and Charged Colloids
AIChE Annual Meeting
Wednesday, October 19, 2011 - 9:45am to 10:10am
Membrane separation processes are increasingly being used in a vast array of industries as a single treatment paradigm for separation of colloids, macromolecules, solutes, and ions. This is either achieved through a sequence of progressively lower cut-off (pore size) membranes, or through use of a process like nanofiltration, whereby a single unit operation is used to remove multiple molecular weight and size species. The major operating costs associated with membranes include creating the pressure driving force for permeation, and mitigation of fouling. Both these operating costs are directly related to the overall energy consumption of membrane filtration.
Considerable attention has been devoted toward mitigating fouling during membrane filtration. Notably, fouling by colloids and large macromolecules is traditionally viewed as formation of a secondary permeable layer that deposits on the membrane, and increases the overall resistance toward solvent permeation. Over the past three decades, most studies on fouling have typically ignored the concept of microscale manipulation of the fouling deposit itself to modify its permeability, or its ability to selectively adsorb ions or small organic contaminants. This is obvious, because the main goal of fouling mitigation has been to prevent the formation of any fouling layer on the membrane. Only in limited cases of pre-coat based processes has the concept of forming a fouling layer on top of a membrane been utilized.
Our research on use of membranes for produced water treatment has led us to consider membrane fouling arising from a combination of colloids (clays and silica), organics, and ions. In this context, we are exploring techniques of manipulating the structure of a colloidal fouling layer using electrostatic fields. Such manipulation of the meso-scale structure can alter the porosity, adsorption of different species, and salt rejection. In this paper, we present a theoretical study on development of a coupled model that addresses the electrokinetic affect on the permeability of a bed of spherical colloids employing a sphere-in cell model. Combined with the electrostatic charge of the colloids, the layer can reduce the transport of ions, effectively enhancing the rejection of salt electrokinetically. A coupled model of concentration polarization, cake layer formation by charged colloidal particles and ion transport through the interstitial spaces of cake layer has been developed to predict the permeate flux decline and salt rejection due to fouling during cross flow nano-filtration of an electrolyte and colloidal suspension. The model considers transport of finite size ions around the charged colloidal particles of the cake layer using a perturbation analysis of the governing Poisson-Nernst Planck equations in light of classical models of electrophoresis in concentrated dispersions. The suspension structure of the cake layer is considered as swarm of colloidal particles and is represented using the Kuwabara cell model, with the inclusion of electrophoresis analysis based on Levine-Neale model. Electrophoresis analysis is then combined with the standard film theory of cross flow membrane filtration to predict the flux decline and rejection. The model illustrates the combined influence of colloid mass transfer coefficient, electrical and physical properties of colloids and electrolytes, and membrane resistance on normalized permeate flux, observed rejection and osmotic pressure.