Salt Concentration at Phase Boundaries in Desalination by Reverse Osmosis

Polyamide desalination membrane characterization and surface modification to enhance fouling resistance.

“…Three main surface properties are thought to influence surface fouling of reverse osmosis membranes: r oughness, hydrophilicity, a nd e lectrostatic c harge [22]. P olyamide reverse osmosis m embranes ha ve r ough, pe aks-and-valleys s urfaces, w ith a n average s urface roughness on the order of 100 nm, and a maximum peak to valley distance of nearly 500 nm [23,24] . T he v alleys o f t his r ough m embrane s urface c an catch s mall p articles, blocking surface ar ea an d thereby reducing w ater f lux [25].…”

“…In t he t apping m ode, short, rapid tapping of the tip on t he membrane surface is used to map the surface. The tapping mode is preferred for soft samples that may be easily deformed by the tip (e.g., polyamide membranes) [23].…”

“…A m ass t ransfer co rrelation an alysis performed using t he e xperimental data a lso i ndicated t urbulent f low c onditions, a s i s e xplained further below. Additionally, the flow was not fully developed because the channel length is only 82 m m. The feed was continuously passed through the carbon and particle filter (to maintain more constant performance during the course of the experiments, as will be explained further below), and feed pH was buffered to the manufacturer-suggested value (pH ~ 8 [8,21]) using NaHCO 3 , to mimic the natural adsorption of CO 2 from air [22,23]. The concentration of NaHCO 3 required to achieve these pH values contributes negligibly to the osmotic pressure (5.0×10 -4 M (0.025 bar)).…”

“…These results suggest in feeds containing an emulsion, modified XLE membranes have similar or better fouling resistance compared to modified NF90 membranes, with less negative impact on NaCl rejection. During fouling, deposition of a foulant layer on the membrane surface may further increase the surface N aCl co ncentration ( i.e., be yond t he i ncrease r esulting f rom c oncentration pol arization [23][24][25]), due t o hi ndered ba ck di ffusion of N aCl t hrough t he f oulant l ayer t o t he bul k f eed solution [22]. This phenomenon, referred to as cake-enhanced osmotic pressure, leads to higher NaCl co ncentration at t he m embrane s urface, higher s alt f lux t hrough t he m embrane, a nd, therefore, l ower N aCl r ejection [22].…”

Mass transfer in plane and square ducts

“…Work on mass/heat transfer have also been primarily concerned with planar geometry, often for suction; of interest in membrane science. Sherwood et al [8][9][10] considered mass transfer for reverse osmosis based on [2] (for mathematical details see Appendix A). Numerical studies of heat and mass transfer have also been reported [11][12][13][14][15][16][17][18][19].…”