Experimental and Computational Fluid Dynamics Investigation of the Deterioration of the Rejection Capacity of the Membranes Used in the Filtration of Oily Water Systems

Abstract

Pressure-driven filtration using membrane technology holds promise for the treatment of produced water but is restricted by the problem of fouling. Fouling is the accumulation of dispersed materials at the surface of the membrane, thereby decreasing the area available for filtration and decreasing the permeation flux. While this is intuitively understandable when the dispersals are solid particles, it is less clear when the dispersals are droplets of other fluid immiscible with the continuous one. Fluids can deform, squeeze, and take the shape of the pore. This questions the proposition and practice of many researchers to extend the blocking mechanisms, originally developed for the filtration of solid emulsions, to the filtration of fluid emulsions. To explore this topic, in this work, we perform experimental and numerical studies to investigate whether the blocking laws can still apply to the filtration of fluid emulsions. The experimental setup involves the use of three types of polymeric microfiltration membranes as well as three different oils. The results obtained show that the oil content in the permeate increases with time contrary to what would be the case if the dispersals were solid particles. Furthermore, the rejection capacity of the membrane decreases with time, implying that more oil permeates as fouling develops. Again, this is contrary to the observed practices in the filtration of solid emulsions where the solid content in the permeate also decreases as fouling develops. To confirm this behavior, three scenarios were investigated numerically using the tools of computational fluid dynamics. The numerical study highlights that, when a pore opening is filled with oil, larger droplets that would otherwise not permeate experience permeation. This enforces the understanding that, as fouling develops, the pores that are filled or blanketed with oil do not restrict oil from permeation; rather, they ease its movement and decrease the selectivity of the membrane. The filtration of fluid emulsions is fundamentally different from that of solid emulsions. Fluids can deform, squeeze, and take the shape of the pores. In this case, interfacial tension forces provide the criterion that defines the selectivity of the membranes.

Recent Aspects in Membrane Separation for Oil/Water Emulsion

“…Oil concentration in water permeate mainly depends upon feed wastewater-oil concentration with its nature either dissolved or dispersed oil. [192] It is governed by the oil-water ratio, total dissolved solids, salinity, pH, temperature, and other dissolved pollutants in OWW. [193] Membrane fabrication, which is cost-effective UF and fouling resistant, for delivery required water quality either for discharge (from 10 to 15 ppm oil) or even less oil concentration for reuse considering the vast variation in oil-water emulsion concentration according to the origin is a challenging task.…”

A Unified, One Fluid Model for the Drag of Fluid and Solid Dispersals by Permeate Flux towards a Membrane Surface

“…This significantly reduces the computational burden to fractions of those required when accounting for both interfacial forces and interface configuration. In several of our previous CFD investigations of the problem of permeation of a microscale oil droplet, a very small time step is required (in the order of 0.01 µs) to achieve the required tolerance with the total time of only fractions of a second [2,3,[10][11][12][13][14]. In this present technique, the time step and spatial discretization can become considerably coarse compared with interfaceresolving techniques.…”

“…The problem of fouling is considered the most serious problem that can hinder the operational effectiveness of membranes and considerably shorten their lifetime. Therefore, there are a large amount of research works that have been devoted to investigate this phenomenon, highlight the factors that may affect its development, and propose methodologies to combat its progression [1,2]. Fouling is the outcome of two phenomena-namely, (1) the selectivity feature of the membrane and (2) the hydrodynamic drag that dispersals encounter towards the membrane surface [2][3][4][5][6][7][8], to list but a few.…”

“…Therefore, there are a large amount of research works that have been devoted to investigate this phenomenon, highlight the factors that may affect its development, and propose methodologies to combat its progression [1,2]. Fouling is the outcome of two phenomena-namely, (1) the selectivity feature of the membrane and (2) the hydrodynamic drag that dispersals encounter towards the membrane surface [2][3][4][5][6][7][8], to list but a few. The selectivity feature of membranes is essentially related to the sizes of their pores in relation to the sizes of dispersals.…”

“…In other words, even at the scale of a single droplet, there are still issues related to the correct representation of the drag force over a wider range of Reynolds numbers along with different viscosity contrasts. The traditional methods for handling this problem involve a sharp interface and diffuse interface models that are very expensive computationally and require exhaustively denser meshes [2,3,[10][11][12][13][14]. In this work, we introduce an algorithm that may work not only to study the drag over solid particles but also that over droplets.…”