Theoretical and experimental approaches of liquid entry pressure determination in membrane distillation processes (2025)

Omniphobic Hollow-Fiber Membranes for Vacuum Membrane Distillation

Characterization of hydrophobic polymeric membranes for membrane distillation process

International Journal of Material Forming, 2010

Hydrophobic microporous membranes are utilized in membrane distillation (MD) processes, e.g. seawater desalination at moderate temperatures. The vapour permeability of commercial hydrophobic membranes with different pore sizes (0.2-1 micron) was characterized through a simple apparatus designed-on-purpose. A cylindrical vessel had a face closed by the membrane and the other connected to a thin graduate tube. The water level variation in the tube is recorded and related to the vapour flux across the membrane. Measurements were taken in the temperature range 20-80°C. A fan tangential to membrane surface was employed to maintain a constant driving force for vapour transport. Vapour flux did not depend on pore dimension, but the membrane and support material resulted to influence the mass transfer. Moreover, the results showed that the main resistance for mass transfer is located in the permeate side, thus addressing future works on the set up of a MD pilot unit.

Effect of spinning conditions on the structure and performance of hydrophobic PVDF hollow fiber membranes for membrane distillation

Desalination, 2012

Microporous hydrophobic poly(vinylidene fluoride) (PVDF) hollow fibers were prepared via dry-jet wet phase-inversion method using N,N-dimethylacetamide (DMAc) as solvent, LiCl and PEG-400 as nonsolvent additives in the polymer dopes. The effects of various preparation conditions, including the concentration of polymer and additives, bore liquid temperature, air gap, take-up speed and dope extrusion rate on the morphology and properties of membrane were studied. The prepared membranes were characterized through scanning electron microscopy (SEM) observation, gas permeation measurement, and tensile property test. The permeate flux of membrane in both vacuum membrane distillation (VMD) and direct contact membrane distillation (DCMD) for desalination was tested. It is found that the permeation property of membrane is mainly determined by membrane porosity, especially effective porosity. The formation of the sponge structure reduced VMD flux more fiercely than DCMD flux. Under the synergetic effect of PEG-400/LiCl, high permeate flux and relatively high mechanical strength of membrane can be simultaneously achieved. There exist the best ranges of bore liquid temperature and of air gap distance for relatively high permeate flux of membrane.

Novel PVDF hollow fiber membranes for vacuum and direct contact membrane distillation applications

Separation and Purification Technology, 2013

Microporous, hydrophobic hollow fiber membranes were prepared from polyvinylidene fluoride (PVDF) under different processing conditions and by changing the composition of the polymeric dope. In particular, different additives were incorporated into the dope (Polyvinyl Pyrrolidone and Maleic Anhydride) and the molecular weight and the concentration of PVDF polymer into the dope solution (medium and high Mw, 15-18 wt.%) were varied, in order to obtain fibers with different morphologies. The prepared membranes were characterized in terms of morphology, mechanical properties, bubble point, overall porosity, pore size distribution and thickness. Comparison between their performance in membrane distillation (MD), working under vacuum and direct contact configurations was also performed. The transmembrane flux obtained was related to the membrane properties, which are function of the operating spinning conditions and the polymeric dope composition. This systematic investigation is an attempt to establish a correlation between the MD performance (in terms of trans-membrane flux) and the hollow fiber preparation conditions (polymeric dope composition and spinning conditions).

Influence of Pore Structure on Membrane Wettability in Membrane Distillation

Amongst the vast variety of desalination technologies suitable to be coupled with solar energy, membrane distillation (MD) is recently being considered because of its feasible features for stand-alone systems and low operational requirements. MD is a thermally driven separation process that uses a hydrophobic membrane to create a liquid-vapor interface from which water evaporates. This vapor crosses the membrane and is condensed on the other side as salt-free distillate. One problem that faces MD systems is premature wetting of the membrane, leading to operational interruptions. Pore wetting happens when feed liquid (seawater in desalination application) is subjected to pressure high enough to cause it to pass through the pores of the hydrophobic membrane. This pressure is termed Liquid Entry Pressure (LEP). Membrane wetting can be predicted using a basic force balance equation. However, MD operations have proven to be more complicated than what can be predicted for a membrane with simple cylindrical pores. In this study, we studied the impact of pore structure on pore wetting in MD membranes. Several interesting trends were observed and discussed. The ultimate goal of this research is to make better prediction of the conditions at which the membrane will start to wet and fail. This will enable a more sustainable MD operation.

Surface Modification of Polytetrafluoroethylene Hollow Fiber Membrane for Direct Contact Membrane Distillation through Low-Density Polyethylene Solution Coating

ACS Omega

Membrane distillation (MD) is an attractive technology for the separation of highly saline water used with a polytetrafluoroethylene (PTFE) hollow fiber (HF) membrane. A hydrophobic coating of lowdensity polyethylene (LDPE) coats the outer surface of the PTFE membrane to resolve membrane wetting as well as increase membrane permeability flux and salt rejection, a critical problem regarding the MD process. LDPE concentrations in coating solution have been studied and optimized. Consequently, the LDPE layer altered membrane morphology by forming a fine nanostructure on the membrane surface that created a hydrophobic layer, a high roughness of membrane, and a uniform LDPE network. The membrane coated with different concentrations of LDPE exhibited high water contact angles of 135.14 ± 0.24 and 138.08 ± 0.01°f or membranes M-3 and M-4, respectively, compared to the pristine membrane. In addition, the liquid entry pressure values of LDPE-incorporated PTFE HF membranes (M-1 to M-5) were higher than that of the uncoated membrane (M-0) with a small decrease in the percentage of porosity. The M-3 and M-4 membranes demonstrated higher flux values of 4.12 and 3.3 L m −2 h −1 at 70°C, respectively. On the other hand, the water permeation flux of 1.95 L m −2 h −1 for M-5 further decreased when LDPE concentration is increased.

Porous Hydrophobic–Hydrophilic Composite Hollow Fiber and Flat Membranes Prepared by Plasma Polymerization for Direct Contact Membrane Distillation

Membranes, 2021

High water vapor flux at low brine temperatures without surface fouling is needed in membrane distillation-based desalination. Brine crossflow over surface-modified hydrophobic hollow fiber membranes (HFMs) yielded fouling-free operation with supersaturated solutions of scaling salts and their precipitates. Surface modification involved an ultrathin porous polyfluorosiloxane or polysiloxane coating deposited on the outside of porous polypropylene (PP) HFMs by plasma polymerization. The outside of hydrophilic MicroPES HFMs of polyethersulfone was also coated by an ultrathin coating of porous plasma-polymerized polyfluorosiloxane or polysiloxane rendering the surface hydrophobic. Direct contact membrane distillation-based desalination performances of these HFMs were determined and compared with porous PP-based HFMs. Salt concentrations of 1, 10, and 20 wt% were used. Leak rates were determined at low pressures. Surface and cross-sections of two kinds of coated HFMs were investigated b...

Characterisation of three hydrophobic porous membranes used in membrane distillation Modelling and evaluation of their water vapour permeabilities

Journal of Membrane Science, 2002

Pore size distributions have been obtained for three hydrophobic porous membranes from air-liquid displacement measurements, assuming the common model of cylindrical capillaries for the membrane and using flux equations which includes both diffusive and viscous mechanisms for transport in the gas phase in pores. The pore size distribution so obtained and the same flux equations are used in order to predict the water vapour permeability through the membranes, characterised when employed in different membrane distillation configurations and operating conditions. In membrane distillation applications, where no viscous flux exists, the role of both Knudsen and molecular diffusion resistances is analysed. In membrane distillation applications which include diffusive and viscous transport, the contribution of both mechanisms is analysed.

Water as the Pore Former in the Synthesis of Hydrophobic PVDF Flat Sheet Membranes for Use in Membrane Distillation

Hydro Science & Marine Engineering, 2019

Although PVDF flat sheet membranes have been widely tested in MD, their synthesis and modifications currently require increased use of green and inexpensive materials. In this study, flat sheet PVDF membranes were synthesized using phase inversion and water as the pore former. Remarkably, the water added in the casting solution improved the membrane pore sizes; where the maximum pore size was 0.58 µm. Also, the incorporation of f-SiO2NPs in the membrane matrix considerably enhanced the membrane hydrophobicity. Specifically, the membrane contact angles increased from 96° to 153°. Additionally, other parameters investigated were mechanical strength and liquid entry pressure (LEP). The maximum recorded values were 2.26 MPa and 239 kPa, respectively. The modified membranes (i.e., using water as the pore former and f-SiO2NPs) were the most efficient, showing maximum salt rejection of 99.9% and water flux of 11.6 LMH; thus, indicating their capability to be used as efficient materials for...

ENHANCEMENT THE FLUX OF PVDF-CO-HFP HOLLOW FIBER MEMBRANES FOR DIRECT CONTACT MEMBRANE DISTILLATION APPLICATIONS

Phase inversion technique has been utilized to prepare Poly (vinylidene fluoride-co-hexafluoropropylene) PVDF-co-HFP, hollow fiber membranes. Polyvinylpyrrolidone (PVP) with 9 wt. %. added as a pore former additives to the polymer dope solution. Characteristics of the PVDF-co-HFP hollow fiber membrane with / without PVP particles have been studied. It was found that the membrane prepared without PVP additives has a low porosity and a high contact angle. Existence the PVP additives of 9 wt. % causing the increase of the membrane porosity by 28 %.Whilst increase PVP content resulting in decrease of membrane hydrophobicity. MD experiment was done using a direct contact membrane distillation (DCMD) configuration as crucial test to investigate performance of product PVDF-co-HFP hollow fiber membrane. Increase the amount of PVP to 9 wt. % in dope solution, this in turn leads to an increased the permeate flux from of 4.5 to 15.8 Kg/m2.h at 70 oC The effect of operating conditions such as feed temperature, concentration of feed solution and permeate flow pattern on the performance flux of the hollow fiber membranes were studied.