Electrospun membrane for distillation

Schematic showing membrane distillation.

Membrane distillation works by having a membrane to separate the salty water and pure water (permeate). There are a few parameters that affect the rate and efficiency of membrane distillation. First, there must be a temperature gradient between the feed side and permeate. Traditionally, the separation layer is relatively thick so that there will be less heat from the feed to permeate, thus maintaining an optimal temperature gradient. The porosity, pore size and tortuosity also affects the ease of water vapour passing through the membrane and to form permeate.

For an electrospun membrane to be used in membrane distillation, it must be able to maintain a separation between the feed and permeate. Thus, a superhydrophobic membrane is preferred as it is better able to maintain water separation. Beaded fibers are known to exhibit higher water contact angle compared to smooth fibers. Electrospun poly(vinylidene fluoride) (PVDF) nanofiber membrane is commonly investigated for use in water filtration. Essalhi and Khayet (2014) investigated the properties of PVDF from beaded to smooth fibers for the purpose of membrane distillation. Although beaded fibers showed a higher water contact angle, the flux from smooth fibers are better than beaded fibers which may be due to the smaller void volume fraction of beaded fibers. However, when used in membrane distillation the PVDF nanofibrous membrane started to be wetted in less than an hour of operation [Liao et al 2013]. Further comparative tests on electrospun nanofibrous membrane and commercially available membrane showed that the liquid entry pressure (LEP) of electrospun PVDF membrane is less than 0.64 bar compared to 9 bar from PTFE membrane [Jiricek et al 2016]. Electrospun PVDF membrane showed greater flux at higher crossflow velocity (80 mm/s) compared PTFE membrane and other commercial membranes. At lower water salinity up to 50 g/kg, thinner membrane has greater permeability and this is where electrospun membrane has an advantage. However, at higher salinity, flux for nanofibrous membrane drops quickly while PTFE membrane showed little changes [Jiricek et al 2016]. The effect of feed salinity on flux was also shown by Jiricek et al (2017) with electrospun polyurethane membrane. However, this is only significant for thinner membrane (6 and 10 g/m². For thinner membranes, initial flux of more than 10 kg/m²/h can be achieved for lower salinity. The flux drops to about 8 kg/m²/h when the salinity increased to 20%. This flux was maintained until salinity reached 70% and the flux drops further. For thicker membranes (25 and 40 g/m²), the flux remains between 6 to 8 kg/m²/h regardless of salinity level. The decline in the performance of thin membrane at increasing salinity has been attributed to its larger pore size and lower bubble point pressure as measured in the study. This may result in easier penetration of water molecule and pore wetting which reduces the flux. To increase the hydrophobicity of the membrane and maintain water separation for longer duration, Liao et al (2013) modified the surface of PVDF nanofibers with a coating of silver nanoparticles through electroless silver-plating followed by a coating of 1-dodecanethiol. The resultant membrane showed superhydrophobicity characteristic with contact angle of 153° and water sliding angle below 10°. The flux of the membrane is relatively unchanged from unmodified PVDF nanofibrous membrane at 32 L/m²/h for the test duration of 8 h. Instead of surface treating nanofibers with a superhydrophobic treatment, Zhou et al (2014) used a commonly known superhydrophobic material polytetrafluoroethylene (PTFE), for fabrication into nanofiber membrane. Since PTFE is resistance to most solvents, they used a suspension of polytetrafluoroethylene (PTFE) fine particles in water for blending with water soluble polyvinyl alcohol (PVA). The PVA with PTFE particles were electrospun and the PVA component removed through sintering up to 380°C for 30 minutes. At this temperature the PTFE particles melted and fused together to form an interconnected nanofibrous network of PTFE. The resultant membrane showed a water contact angle of 156.7°. When tested for vacuum membrane distillation, a pure water flux of 15.8kg/m²h and a stable salt rejection of more than 98% for ten hours were recorded. Ray et al (2018) incorporated Cera flava into electrospun polysulfone (PSF) to increase its hydrophobicity to 162°. The composite nanofibers network was electrospun onto a hydrophobic polypropylene (PP) membrane to form a hybrid membrane. This hybrid membrane was shown to demonstrate salt rejection above 99.8%, and a high permeate flux of approximately 6.4 L m^-2h^-1 was maintained for 16 h of operation based on aqueous NaCl solution with 30 g L^-1 concentration. This flux is consistent with other research using electrospun fibers. However, Ray et al (2018) also showed that after 30 h of usage, there was only a 3.1% decrease in initial flux.

A limitation of electrospun membrane is its low liquid entry pressure of water which may result in pore wetting over time. To address this issue of pore wetting, Prince et al (2014) used a triple layer membrane comprising of a hydrophilic nanofiber base (in touch with the permeate), a cast membrane middle layer and an electrospun PVDF top layer (in touch with the feed). The function of the nanofibrous PVDF top layer is to present a superhydrophobic surface to keep the water from entering the membrane. The cast membrane middle layer with its smaller pore size will increase the liquid entry pressure. The lower hydrophilic layer will help to draw water vapour from the middle layer by absorption. The triple membrane layer with optimized thickness was able to maintain a stable flux of 50 to 70 L/m² over 97 h and salt rejections above 99%. Al-Furaiji et al (2019) also tested a triple layered membrane for direct contact membrane distillation. In their membrane configuration, a commercially available polyethersulfone (PES) nanofiber nonwoven material from DuPont was sandwiched between two layers of electrospun polyvinylidene fluoride (PVDF) nanofibers. The PES layer function as a mechanical support for the electrospun membranes. Smaller pore size of the PES layer may also help to increase the liquid entry pressure of water through the membrane. Tests using the PES layer alone showed no flux or salt rejection as the membrane gets wetted out almost immediately. With a single layer of electrospun membrane, the salt reject was almost 100% initially but was reduced significantly after 2 hours due to wetting throughout the membrane. With the triple membrane setup, a rejection rate of almost 100% was maintained for 6 hours at fluxes between 6 and 9 kg/m². An optimum membrane thickness of 115 µm was determined for maximum flux. A thinner membrane would increase the rate of heat transfer by conduction although it provides a higher rate of mass transport. A thicker layer increases mass transport resistance although it reduces the heat transfer.