Elsevier

Separation and Purification Technology

Volume 118, 30 October 2013, Pages 598-603
Separation and Purification Technology

Preparation and characterization of novel triple layer hydrophilic–hydrophobic composite membrane for desalination using air gap membrane distillation

https://doi.org/10.1016/j.seppur.2013.08.006Get rights and content

Highlights

  • Novel triple layer nanofiber-based membranes were prepared.

  • Membrane were tested in an air gap membrane distillation process.

  • Triple layer membranes were compared with dual layer membranes and nanofiber only layers.

  • Nanofiber layer significantly improved flux and pore wetting resistance of membrane.

Abstract

Conventional membrane distillation (MD) membranes typically consist of two layers i.e. a support layer and a microporous hydrophobic layer that is usually casted onto the support layer. In this paper a triple layer configuration, consisting of a thin hydrophobic nanofiber layer over the conventional casted microporous layer in addition to the support layer is prepared. The layers are bound to each other by heat pressing and solvent binding. The unique membrane was characterized using the measurement of the liquid entry pressure of water (LEPw), scanning electron microscopy (SEM), pore size, mat thickness and the surface water contact angle (CA). The triple layer membranes, conventional dual layer membranes and nanofiber only membranes were tested in an Air-Gap Membrane Distillation (AGMD) process using a feed solution containing 3.5 wt.% NaCl. The water contact angle of the nanofiber selective layer, middle micro porous layer and the hydrophilic supportive layers are approximately 145°, 92° and 30° respectively. The LEPw of the triple layer composite membrane was found to be 1.6 times higher than the conventional dual layer membrane (i.e. without the nanofiber selective layer) and more than 8.7 times higher than that of the nanofiber membrane. The fabricated triple layer composite membranes were tested in an AGMD process and the flux was also found to be 1.5 times higher than that for the dual layer membrane (i.e. without nanofiber selective layer). In terms of salt penetration, the triple layer membrane had the lowest salt penetration ⩽0.02% at different feed water temperatures, while the salt penetration of the dual layer membrane was higher at ⩽0.07% and increased with increasing feed water temperature. The nanofiber only membrane’s salt penetration increased dramatically to more than 90% within the first few minutes of filtration indicating severe pore wetting. The triple layer membrane could be operated continuously for more than 40 h without any significant change to permeate quality, whereas the dual layer membrane could only be operated for 10 h before pore wetting occurred. Based on the results it is proposed that the triple layer configuration with nanofiber based selective layer above the conventional dual layer could be a promising membrane for membrane distillation.

Introduction

Membrane distillation (MD) technology is being increasingly investigated for seawater desalination and liquid concentration applications [1], [2]. In terms of seawater desalination, MD has some advantages compared to other conventional desalination processes such as Reverse Osmosis (RO) and distillation. These advantages include high rejection of non-volatile solutes, lower operating temperatures than conventional distillation, lower operating pressures than conventional pressure-driven membrane processes and reduced vapor spaces compared to conventional distillation processes [3].

Membrane distillation despite being introduced since 1963 [4], has not been widely used, with a key limiting factor for the success of MD being the membrane. Generally, MD membranes should be hydrophobic, micro-porous and have high liquid entry pressure. The development of MD membranes with higher flux has been one goal of MD research with the other goal being the reduction of pore-wetting. Pore-wetting which occurs when the liquid stream penetrates into the membrane pores or vapor condensation occurs in the membrane matrix leads to flux decay and salt penetration [5], [6], [7].

Several membrane configurations have been suggested for MD from single layer to dual layer. Composite dual layer MD membranes consisting of a hydrophobic top layer and a hydrophilic sub-layer were patented in the 1980s [Cheng D.Y. and Wiersma S.J. 1983 and 1983, Composite membranes for a membrane distillation system, US Patents 4,316,772 and 4,419,242]. Much research has focused on improving MD membranes by increasing its hydrophobicity. Various methods such as chemical vapor deposition, grafting, stretching, modifying the thermally induced and non solvent induced phase separation methods have been explored for producing hydrophobic surfaces [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19]. Recently, the technique of electrospinning has been harnessed to produce nanofiber mats, which have been suggested for MD [1], [8], [9].Although, various polymers can be prepared for use as nanofiber mats in MD, recent work has focused on PVDF. These mats typically have a high porosity and a highly hydrophobic surface, which make them ideal for MD applications. However, the liquid entry pressure (LEPw) of these nanofiber mats is typically low, which might suggest a higher propensity of pore wetting over time. Recent work showed that a PVDF nanofiber membrane of thickness 0.3 mm had a LEPw of only 90 kPa and salt transmission through the membrane was observed when operated in DCMD mode after 1 h [8]. However it may be possible to optimize this layer further and control the operating conditions so that prolonged use of at least 15 h with little change to the permeate quality is possible despite only a LEPw of 35 kPa [9].However, these nanofiber membranes could potentially encounter pore wetting issues during air-gap membrane distillation (AGMD) and vacuum membrane distillation (VMD), due to the differential pressure across the membrane and at high feed water velocities.

To further enhance the performance of the nanofiber membranes and the conventional dual layer membranes for MD use, we propose a triple layer nanofiber based membrane. Produced by wet casting and electrospinning, each layer within the triple layer configuration serves a specific function.

The top selective layer consisting of electrospun PVDF nanofibers, which are highly hydrophobic are expected to prevent the water molecules from passing through the membranes. The high porosity (70–90%) of the nanofiber layer is postulated to reduce the heat loss across the membrane and be saturated with water vapor. The middle layer will help increase the LEPw of the membrane to prevent long term pore-wetting and the bottom hydrophilic layer will help to draw water vapor from the middle layer by absorption [20], [21], [22], [23]. The triple layer membrane, a dual layer membrane consisting of all the components of the triple layer membrane except for the nanofiber top layer and a single nanofiber layer membrane were prepared, characterized and evaluated in the AGMD mode.

Section snippets

Materials

Polyvinylidene fluoride (PVDF) Kynar® 761 grade with a melting point of 165–172 °C was purchased from Arkema Pte. Ltd., Singapore. Polyvinylpyrrolidone-K17 (PVP-K17) tech grade was purchased from Shanghai Welltone Material Technology Co., Ltd., Shanghai, China. Ethanol, acetone and N,N′ dimethyl acetamide (DMAC) were analytical grade from Sigma, Singapore. The water used was distilled and purified with a Milli-Q plus system from Millipore, Bedford, MA, USA.

Preparation of triple layer composite membranes

Casted PVDF membranes were prepared

Membrane characterization

The SEM images of the top surface of the triple layer membrane, dual layer membrane and electrospun nanofiber membrane were observed and shown in Fig. 2a. The SEM images of the cross-section of the triple layer composite membrane are shown in Fig. 2b. It can be clearly observed from the image that the micro porous casted layer is sandwiched between the nanofiber and the supportive layers. The top layer consisting of the highly porous electrospun nanofiber has a thickness of approximately 25 μm

Conclusions

Novel triple layer hydrophobic/hydrophilic membranes were prepared through wet casting and electrospinning process. The addition of the nanofiber layer on the dual layer membrane was found to increase the flux, salt rejection and long term performance of the membranes. This was due to the increased hydrophobicity and liquid entry pressure of the triple layer membrane over the dual layer and nanofiber single layer membrane. Our own work suggests that the nanofiber membranes are not suitable for

Acknowledgements

We gratefully acknowledge funding support from Singapore’s Ministry of Education under the Innovation Fund (MOE-IF-1-042). Support from SPRING Singapore and Ngee Ann Polytechnic to the Environmental & Water Technology Centre of Innovation is also acknowledged.

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