Elsevier

Journal of Water Process Engineering

Volume 26, December 2018, Pages 124-130
Journal of Water Process Engineering

Inorganic microfiltration membranes incorporated with hydrophilic silica nanoparticles for oil-in-water emulsion separation

https://doi.org/10.1016/j.jwpe.2018.10.002Get rights and content

Highlights

  • Silica nanoparticles were incorporated on the alumina microfiltration membrane.

  • The incorporated silica nanoparticles increased the membrane hydrophilicity.

  • The permeate oil concentration was less than 40 ppm.

  • The oil rejections were stable under varying temperature conditions.

Abstract

Hydrophilic modification of alumina microfiltration membranes was achieved by incorporating silica nanoparticles into the alumina matrix. The alumina tubular membrane, incorporated with silica nanoparticles, was tested for cyclohexane-in-water emulsion separation. The incorporated silica nanoparticles increased the hydrophilicity of the membrane surface with the oil contact angles changing from 155° to 165°. The alumina membrane treated with a 0.5 wt.% silica-nanoparticle solution yielded a water flux greater than 350 L m−2 h-1 and oil rejection greater than 93%, which represented a 20.5% and 6.0% enhancement in water flux and oil rejection compared with the pristine alumina membrane. Due to the hydrophilic silica nanoparticle incorporation, the cyclohexane concentration of the permeate was less than 40 ppm when the cyclohexane concentration of the feed was 500 ppm at 40 °C and feed pressure was 43.7 psia. This work represents a key advancement toward the use of inorganic membranes to treat marginal water that contains hydrocarbon contaminants.

Introduction

On a global scale, approximately three barrels of produced water, a byproduct of oil and gas production containing such impurities as hydrocarbons, dissolved inorganic and organic particles, are produced for every barrel of oil extracted [1,2]. The existence of oil droplets along with surface-active agents eventually tends to form stable oil-in-water emulsions. In order to meet the discharge requirements, < 40 ppm of hydrocarbon in the water phase [3], there is a significant need for efficient, cost-effective strategies to remove hydrocarbons from produced water.

The primary technology used by major producers to remove hydrocarbons from produced water relies upon the addition of chemical demulsifiers to remove surface active agents from the oil-water interface [4]. Chemical demulsifiers have to match with the specific chemical profile of the produced fluid. Due to the lack of a predictive model for choosing appropriate demulsifiers, this process can only be accomplished through inefficient trial-and-error serial testing [4,5]. In addition, the chemical treatments may also result in the formation of sludge, which can cause further pollution [6].

Ultrafiltration (UF) polymeric membranes with pore sizes ranging from 0.01 μm to 0.1 μm are commonly selected to separate water from oil-in-water emulsions by size exclusion. The membrane pore size and membrane thickness can be relatively controlled by selecting appropriate materials. Several studies have been performed on membranes involving oil-in-water emulsions. Yang et al. [7] coated polydopamine (PDA) and polyethyleneimine (PEI) on a polypropylene membrane to control the pore size at around 0.2 μm. An oil rejection of 98% was obtained by optimizing surface structures and properties, which in turn determined filtration performance. Some membranes that were made from natural materials were also studied to realize effective oil/water separations including a bioinspired diatomite membrane and a mineral-coated polypropylene membrane [8,9]. However, the polymeric membrane fabrication process takes extensive time due to strict control of temperature and pressure and complicated procedures such as crosslinking, salt-induced fabrication, and nitrogen atmosphere protection [[10], [11], [12]].

Fouling is a serious issue that causes flux losses of membranes because of contaminants blocking the membrane pores. A typical step for removing foulants on a membrane surface is to wash out the remaining oil from the membrane. Ahmad et al. [13] synthesized a polymeric membrane by utilizing polysulfone, with SiO2 nanoparticles (NPs) for modification. Based on their research, the hydrophilic SiO2 nanoparticles were found to be capable of enlarging pore size with a series of interconnected surface pores. The bigger pores not only increased anti-fouling properties, but also increased membrane permeability simultaneously. Rajasekhar et al. [14] proposed a similar method involving the fabrication of poly-(vinylidene fluoride) (PVDF) and tri-block copolymer (TBC), which are regarded as self-cleaning materials. Different ratios of these two components were investigated with the expectation that membrane properties such as flux, oil rejection and fouling resistance would be enhanced. Yuan et al. [15] also used PVDF membrane to realize a 99% oil rejection rate with the water flux of higher than 650 L/m2-h for the oil-in-water emulsion separation. Although new self-cleaning materials are currently used in polymeric membranes [13,14,16], as high as 70–90% of oil droplets can be deposited and adsorbed on the surface of membrane. This can be counteracted by washing, which can remove as much as 30–40% of oil droplets deposited on the membrane. Polymeric materials however cannot tolerate corrosive condition such as acidic and basic solutions, high temperatures, and organic solvents [[17], [18], [19]]. In addition, swelling due to capillary force or the combination of molecular diffusion and convection in the water-filled pores [20] can make the polymeric membranes ineffective under severe environmental conditions.

Because of the many issues involving polymeric membranes, researchers have focused on ceramic materials in order to overcome the material-based limitations of polymers, as shown in Table 1. Generally, ceramic membranes are synthesized and coated on porous supports such as alumina disks or mullite hollow fibers. Chang et al. [6] modified a ceramic alumina membrane by coating nano-TiO2 on the membrane surface. The pore size of a ceramic alumina membrane without modification was found to be appropriate for oil separation. In addition, TiO2 can increase the membrane hydrophilicity thereby increasing the number of water molecules that are allowed to pass through the membrane, therefore increasing the membrane flux. In some cases, materials are synthesized on the surface [21,22] or in the channel [23] of the supports to enhance transport efficiency and separation selectivity. Chen et al. [23] synthesized the carbon nanotubes in the channels of an yttria-stabilized zirconia (YSZ) membrane to remove tiny oil droplets from the water. Under optimal growth conditions for carbon nanotubes, the oil rejection of ∼100% was obtained. For industrial applications, membrane processes operating under harsh conditions has to be considered when improving the membrane separation efficiency. Studies have been performed about the use of silica for coating microfiltration membranes [[24], [25], [26]]. These silica-decorated membranes not only improved hydrophilicity but also corrosion resistance [27]. Conveniently, another benefit of this modification is that a ceramic support can also protect a membrane by enhancing mechanical stability.

Section snippets

Membrane synthesis

Hydrophilic silica NPs (16 nm, A200, AEROSIL) were added into deionized water (resistivity of 18 mΩ cm−1) and stirred for 2 h to prepare a homogeneous silica-NPs containing solution at the weight percentage of 0.05%, 0.5%, and 5%. The α-alumina tubular membrane (70 mm length, 11 mm outer diameter, 8 mm inner diameter, 0.2 μm pore size, CoorsTek) was washed with ethanol and water successively, and dried in 60 °C overnight. The dried membrane was then immersed into the silica NPs containing

Characterization of the membrane

The morphology of the membranes after silica NPs were incorporated on the alumina membrane is shown in Fig. 3. The silica NPs were incorporated on the surface and in the pore channels of the alumina membrane. When the alumina membrane was treated with a 0.5% solution, the silica NPs were incorporated on the membrane surface (Fig. 3e-g) which resulted in minimal blocking of membrane channels (Fig. 3h). However, when the 5% solution was used, a continuous layer of silica NPs was formed on the

Conclusion

We demonstrated a novel method to incorporate hydrophilic silica NPs into an α-alumina microfiltration tubular membrane for oil-in-water emulsion separation. After treating the alumina membrane with a 0.5 wt.% silica NPs precursor solution, the water flux was greater than 350 Lm−2 h-1 and the oil rejection was greater than 93%. This represents a 20.5% and 6.0% enhancement in the flux and the oil rejection compared with the unmodified membrane. The contact angles of oil on the modified membranes

Funding

This work was supported by Oklahoma State University; the National Energy Solutions Institute – Smart Energy Source (NESI-SES); and the Technology and Business Development Program (TBDP).

Conflicts of interest

None.

Acknowledgement

We thank Dr. Nicholas Materer and Dr. David Jacobs from the Department of Chemistry, Oklahoma State University, for granting us access to the GC/MS and for their help with the GC/MS experiments.

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