Fabrication of hydrophobic PP/CH₃SiO₂ composite hollow fiber membrane for membrane contactor application

Highlights

• Hydrophobic silica nanoparticles (CH₃SiO₂ NPs) was synthesized by sol-gel process.

• Nanocomposite hydrophobic polypropylene hollow fiber membranes were fabricated via two treatment approaches.

• Membrane mass transfer resistance was decreased significantly in the presence of CH₃SiO₂ NPs.

• CO₂ absorption flux of the nanocomposite membranes was stable within 30 days operation time.

Abstract

Polymeric membrane wetting by liquid absorbent is one of the major challenges of the gas- liquid membrane contactors. In order to overcome this issue, composite polypropylene (PP) hollow fiber membranes were prepared by incorporation of methyl grafted silica nanoparticles (CH₃-g-silica NPs) via two treatment approaches including blending the NPs with dope solution before membrane fabricating (approach 1), and also coating the NPs on the surface of the prepared membrane (approach 2). The fabricated membranes were characterized by using ATR-FTIR, FESEM, TEM, AFM, porosity, contact angle, mechanical strength, breakthrough pressure, wetting resistance and gas permeation test. The obtained results from ATR-FTIR spectra confirmed the successful formation of the PP-CH₃SiO₂ composite hollow fiber membranes. It was seen that for composite membranes fabricated with approaches 1 and 2 the average contact angle increased from 124° to 145° and 168°, respectively. The fabricated membranes were also investigated for CO₂ absorption process in gas- liquid membrane contactors. As a result, the gentle decrease in flux of composite membranes compare to the sharp flux drop of neat one confirmed the effective improvement in the non-wetting property of the membranes in the presence of inorganic particles.

Introduction

It is vital to introduce devise techniques which would reduce acid gases like CO₂ arising from the combustion of fossil fuels, present in the natural gas and industrial exhaust due to direct contribution in the greenhouse effects and climate change [1], [2], [3], [4]. The most common process for removal of acid gases is an absorption into a solvent using conventional gas–liquid contactors such as packed towers, bubble columns, spray towers, etc. However, these conventional techniques are energy-consuming and not easy to operate because of some operating problems including flooding, foaming, entrainment and channeling [5], [6]. A promising alternative technology which can overcome these disadvantages and possesses a high potential to replace conventional equipment is membrane contactor. This technology has numerous of benefits, such as flexibility of operation, high surface area per unit volume, being easy to scale up or down and predictable performance module [7], [8], [9]. Due to its excellent mass-transfer properties, membrane gas-absorption technology has been known as a technically viable option by the International Energy Agency's working group on CO₂ removal [10]. However, the membrane contactor suffers from membrane wetting in a long-term operation, which will lead to a sharp increase in membrane resistance against absorption process [2], [11], [12]. Mavroudi et al. investigated the wetting problem in membrane contactor systems and found that the membrane mass-transfer resistance accounted for 20–50% of the total mass-transfer resistance in the case of liquid-filled membrane pores [13].

It is well-known that the porous hollow fiber membrane is a core element in the membrane contactor device [14]. Accordingly, selecting an appropriate polymeric material is logically the first parameter to complete the requirements for membrane contactor processes. However, various experimental results revealed that polymeric membranes, utilized in the membrane contactors, are limited to short term and mild operating conditions, i.e. low alkalinity, acidity and low temperature applications. Accordingly, many studies have been carried out to improve the chemical, mechanical and thermal stabilities and non-wetting characteristics of conventional polymeric membranes to be used in membrane contactor applications [15].

According to the literature, addition of the inorganic fillers into polymeric membranes, known as “composite membranes”, seems to be a promising strategy to improve the membranes properties and consequently achieve a long-term stable operation [16], [17], [18]. Indeed, composite membranes combine the advantages of both polymeric and inorganic membranes and exhibit interesting membrane properties such as considerable hydrophobicity, great chemical and thermal resistances and excellent adaptation to the severe operating conditions [16], [19], [20].

In general, the composite membranes can be produced by two methods, including blending the inorganic particles with dope solution before membrane fabricating, and also coating the inorganic particles on the surface of the prepared membrane [21].

In the case of composite membranes fabricated via first method, Rezaei et al. fabricated Polyvinylideneflouride (PVDF) hollow fiber mixed matrix membranes (MMMs) through wet phase inversion method using general MMT and Cloisite 15A as inorganic fillers to be used in a membrane contactor process [15]. They showed that the permeation flux and wetting resistance of the membranes in terms of contact angle and liquid entry pressure of water increased significantly by filler loading. Taghaddosi et al. prepared nanoclays embedded PP membranes by using thermally induced phase separation (TIPS) method to improve the anti-fouling properties of PP membrane [22]. The obtained results showed that incorporation of nanoclays into PP membranes could significantly mitigate fouling and improve the membranes performance.

Among the inorganic fillers, silica NPs have received much attention due to their well-defined ordered structure, cost-effective production, high surface area and easy surface modification [23]. In the field of composite membranes prepared via coating the silica NPs on the membrane surface, Zhang et al. reported an appropriate method to form a highly hydrophobic Polyetherimide (PEI) membrane [21]. It was observed that incorporation of the fluorinated silica inorganic layer on the membrane surface offered high hydrophobicity and would protect the polymeric membrane from attacks of the chemical absorbents. Wang et al. fabricated PP composite membranes by loading SiO₂ NPs and attaching 1H,1H,2H,2H-perfluorodecyltriethoxysilane (PFDTS) on the membrane surface to be used in vacuum membrane distillation (VMD) [24]. The resulted composite membranes showed good anti-wetting ability and stable anti-fouling performance during the long-term operation. Xu et al. fabricated super-hydrophobic PP hollow fiber membrane by using 1H,1H,2H,2H-perfluorooctyltriethoxysilane (POTS) for membrane distillation (MD) application [25].

There is a wealth of literature available on the utilization of PP composite membranes for wide application; however, there is a lack of literature reporting about application of PP composite membranes as a membrane contactor. PP has excellent characteristics such as high melting temperature, low production cost, excellent processability, widely adjustable mechanical properties, high resistance to water and chemical environments and so on [26]. Due to economic concerns, further expansion of the application area of PP composite membranes is highly desired, thus, in this work, two treatment approaches, described above, were employed fabricate PP composite membranes by incorporation of methyl grafted silica nanoparticles.

In the present work, inorganic NPs were synthesized via sol-gel method. It should be noted that the influential parameters in fabrication of silica NPs via sol-gel method were optimized in our previous work using central composite design of the response surface method (RSM) [18], and the obtained results were used in this work. The fabricated membranes were characterized by various characterization analyses, including ATR-FTIR, FESEM, TEM, AFM, porosity, contact angle, mechanical strength, breakthrough pressure and gas permeation test. Moreover, in order to investigate the wetting resistance of the neat and composite hollow fiber membranes, prepared membranes were tested in a gas-liquid hollow fiber membrane contactor for CO₂ absorption.