Omniphobic Membrane for Robust Membrane Distillation

Abstract

In this work, we fabricate an omniphobic microporous membrane for membrane distillation (MD) by modifying a hydrophilic glass fiber membrane with silica nanoparticles followed by surface fluorination and polymer coating. The modified glass fiber membrane exhibits an antiwetting property not only against water but also against low surface tension organic solvents that easily wet a hydrophobic polytetrafluoroethylene (PTFE) membrane that is commonly used in MD applications. By comparing the performance of the PTFE and omniphobic membranes in direct contact MD experiments in the presence of a surfactant (sodium dodecyl sulfate, SDS), we show that SDS wets the hydrophobic PTFE membrane but not the omniphobic membrane. Our results suggest that omniphobic membranes are critical for MD applications with feed waters containing surface active species, such as oil and gas produced water, to prevent membrane pore wetting. ■ INTRODUCTIONMembrane distillation (MD) is a thermal separation process using a microporous hydrophobic membrane. 1−3 MD can operate at relatively low temperatures and is thus able to tap into the vast amount of low-grade waste heat. 4−6 MD is also advantageous over pressure-driven membrane processes, such as reverse osmosis (RO) or nanofiltration, as its low operating pressure reduces the capital cost due to the absence of expensive components, such as high pressure pumps and vessels, as well as pressure exchangers. Recently, MD has been proposed as a low-temperature thermal separation component for hybrid membrane processes coupled with forward osmosis for simultaneous wastewater reuse and mineral recovery 7,8 and with pressure retarded osmosis for harvesting low-grade waste heat. 9 Although MD, as any thermal separation process, is inherently less energy efficient than RO, 10,11 there exist scenarios in which MD may be preferred. For example, if an abundant amount of waste heat or solar thermal energy is readily available, MD can be employed to utilize such low-grade heat to considerably reduce the energy cost and carbon footprint for desalination compared to RO powered by conventional energy sources. 12−14 MD can also be used to desalinate high salinity brines, such as shale gas wastewater, as the osmotic pressure of such brines is far beyond the allowable pressure in RO operations. 15 In addition, MD can be employed for small-scale desalination in remote regions for which RO is not an option due to its dependence on grid power and costly high-pressure components that are not readily adaptable for small-scale systems.In MD desalination, a hydrophobic membrane is employed to create a vapor gap that separates a salty feed solution and the desalted permeate solution. 16 It is critically important that the membrane pores are not wetted by the feed solution as liquid flooding of the pores destroys the vapor gap and undermines the function of the membrane as a selective barrier for salt passage. 1,17,18 Preventing pore wetting is particularly challenging in desalinating shale gas wastewater or ot...

Tailoring surface charge and wetting property for robust oil-fouling mitigation in membrane distillation

“…To facilitate the detection of membrane wetting, the flowrates of feed and distillate streams were controlled to be 0.45 L/min and 0.2 L/min (9.6 cm/s and 4.3 cm/s in our test cell), respectively [13,18]. This operation conditions resulted in a slightly higher hydraulic pressure in the feed stream than in the distillate stream in this specific test cell, so that the liquid flow through any potentially wetted pores would always be driven from the feed stream to the distillate stream and thereby increase the salinity of the distillate stream.…”

“…Such an advantage, together with the small system footprint and low capital cost, renders MD an appealing process for sustainable and off-grid desalination [7][8][9][10][11]. On the other hand, MD has also been identified as one of the only few promising technological candidates for desalinating shale gas produced water which, in certain cases, is too saline for reverse osmosis to treat [12][13][14].…”

Wetting prevention in membrane distillation through superhydrophobicity and recharging an air layer on the membrane surface

“…Some authors have investigated the improvement of hydrophobic properties of membranes employing novel materials or applying surface modification through manipulating surface chemistry and surface geometry by nanoparticle coating and surface fluorination [21][22][23][24][25][26][27]. However, these membranes are still susceptible to pore wetting when treating feed solutions containing a high concentration of surface-active species.…”

Ultrasonic irradiation control of silica fouling during membrane distillation process

“…Being different from conventional evaporation, MD can operate at relatively low temperatures and is thus able to tap into the vast amount of low-grade waste heat [1]. MD is also advantageous over conventional pressure-driven membrane processes, such as nanofiltration (NF) or reverse osmosis (RO), as its low operating pressure reduces the capital cost due to the absence of expensive components, such as high pressure pumps and vessels, as well as pressure exchangers [2]. According to the condensation method adopted, the MD systems can be classified into four different categories: direct contact membrane distillation (DCMD), air gap membrane distillation (AGMD), sweeping gas membrane distillation (SGMD) and vacuum membrane distillation (VMD) [3].…”