Intensification of O2/N2 separation by novel magnetically aligned carbonyl iron powders /polysulfone magnetic mixed matrix membranes
Graphical abstract
Introduction
Oxygen and nitrogen are highly demanded in various chemical and medical industries. Oxygen is used to improve the efficiency of numerous chemical processes such as natural gas combustion, glass production, coal gasification, sewage treatment as well as welding. Nitrogen is also used as a low-temperature coolant, for the purging of pipelines and vessels, blanketing of equipment, in production of ammonia, and the pharmaceutical industry [[1], [2], [3], [4], [5]].
There are three methods for O2/N2 separation, namely cryogenic distillation, pressure swing absorption and membrane technology. Compared to energy intensive technology as cryogenic distillation and absorption, membrane separation technology has potential for O2/N2 separation due to its low operational costs, relatively small footprint, low energy requirement and ease of processing [[6], [7], [8], [9], [10]]. Moreover, membrane technologies can be combined with other separation treatment to optimize the overall process [11].
The majority of commercially available polymeric membranes for O2/N2 separation are made of glassy polymers such as polysulfone (PSf), polyimide, polyamide, polyurethane, polydimethylsiloxane and their derivatives [[11], [12], [13], [14], [15]]. PSf possesses desirable chemical and physical properties in addition to being cost effective for large scale fabrications [[16], [17], [18], [19]] and since it is currently commercialized for O2/N2 separation, it is widely explored by many researchers for further improvements [20].
Mainly, gas permeation in polymeric membranes is based on the solution-diffusion mechanism, which consists of two parts: sorption of gas molecules into the membrane matrix and then diffusion through the tiny inter-chain polymer spaces. However, the process of O2/N2 separation is not trivial due to the negligible difference (0.18 ) between the kinetic diameters of oxygen and nitrogen, as well as the low solubility selectivity offered by the majority of the membranes [8,21]. Moreover, the trade-off between permeability and selectivity has always been a limiting factor [[22], [23], [24]]. Therefore, in order to develop high performance membranes and processes for O2/N2 separation, it is essential to improve both permeability and selectivity via the established techniques such as designing new polymers, blending two polymers with various physical and chemical properties [9,[24], [25], [26]] or preparation of MMMs among others [4,17,[27], [28], [29], [30]].
Over the last decade, some researchers have focused on the idea of developing magnetic membranes based on the notion of exploiting distinct magnetic properties of oxygen (paramagnetic) and nitrogen (diamagnetic) [31,32]. Oxygen is a paramagnetic molecule having a magnetic moment of μO2 = 2.73 × 10−23 JT-1 and its positive mass magnetic susceptibility is 43.34 × 10-6 m3 kg-1. In contrast, nitrogen is a diamagnetic molecule with a magnetic moment of μN2 = 2.5 × 10-28 JT-1 having negative mass magnetic susceptibility of -15.08 × 10-8 m3 kg-1 at room temperature. It is shown that these properties resulted in spectacular behaviors for oxygen and nitrogen when exposed to any magnetic field [33,34]. This notion can be exploited for improving the separation performance by forming magnetic mixed matrix membranes (MMMs) through introducing magnetic particles into the membrane matrix as well as through conducting the separation in the presentence of the magnetic field [35].
It is hypothesized that magnetic membranes provide extra driving force during the transport of species in addition to the conventional solution-diffusion mechanism existing in the dense polymeric membranes. In magnetic MMMs, the interactions between the paramagnetic oxygen molecules and magnetic channels cause oxygen molecules to pass through the magnetic channels. Magnetic channels are defined as the interfacial voids between particles and polymer, which are created by the agglomeration of particles in the polymer matrix during the membrane preparation process in the presence of magnetic field [36,37]. However, diamagnetic nitrogen molecules escape from the magnetic channels and are diverted to permeate through the polymer matrix. This enables oxygen molecules to transport faster in the magnetic channels in comparison to diamagnetic nitrogen molecules resulting in increased O2 permeability and subsequently improved O2/N2 selectivity.
It should be noted that the specifications of magnetic channel and intensity of interactions between the gas molecules and magnetic channels play significant roles in the separation performance. In a research study, Rybak et al. [38] prepared magnetic MMMs by incorporation of neodymium powder MQP-14-12 into ethyl cellulose (EC), linear and hyperbranched polyimide (LPI and HBPI) matrices. They found that magnetic HBPI membranes exhibited the best performance for oxygen enrichment from air (about 62 %) in comparison to magnetic LPI (58 %) and EC (∼37 %) membranes. This was attributed to the stronger magnetic field and the higher amount of magnetic percolation channels in magnetic HBPI membranes in comparison to others. Also, Madaeni et al. [39] prepared magnetic membranes by coating FluidMAG-PAD super-paramagnetic particles on the surface of a polyethersulfone- polydimethylsiloxane composite membranes. They reported that in the presence of an external magnetic field caused by a coil, oxygen and nitrogen permeance were 23.1 and 0 GPU, respectively. This was attributed to the interactions between oxygen as a paramagnetic component and super-paramagnetic particles inside the membrane matrix which led to increase in oxygen permeance.
Over the past decades, controlling the arrangement of particles in the magnetic composites has been subjected to attention in various fields such as biology, microfluidics, electronics [40], gas separation [[41], [42], [43]] and packaging [44]. For example, Zhu et al. [41] prepared magnetic MMMs by incorporation of Fe3O4– graphene oxide particles into the Pebax and investigated the effect of the arrangement of particles on the separation of CO2, CH4 and N2. The higher CO2 permeability of MMMs prepared under the vertical magnetic field in comparison to those prepared under the horizontal magnetic field was attributed to the creation of shorter transport pathway for gas transport. In a recent study, Rybak et al. [42] prepared iron-encapsulated multi-wall carbon nanotubes (Fe@MWCNTs)/poly(2,6-dimethyl-1,4-phenyleneoxide)(PPO) magnetic MMMs in the presence of magnetic field for O2/N2 separation. They demonstrated that the arrangement of particles in the cross section of membranes was an effective method to create shorter transfer pathways for the gas molecules in the membrane. These pathways can act as a barrier for diamagnetic nitrogen molecules. Moreover, the interaction between oxygen molecules and magnetic pathways, as well as the smaller kinetic diameter of O2 in comparison to N2, led to increasing in O2 permeability and O2/N2 selectivity in the magnetically cast membranes. Therefore, the creation of preferential permeation pathways for paramagnetic oxygen molecules can be regarded a promising method to improve the separation performance of membranes.
In several investigations carried out at Rybak research group [[35], [36], [37], [38],[45], [46], [47], [48], [49]] magnetic MMMs were prepared from diverse polymers, including EC, PPO, LPI and HBPI using different magnetic particles such as praseodymium, neodymium and Fe3O4 in the presence of magnetic field. They investigated the effects of type of particles and polymer as well as the amount of magnetic particle and their size on the O2/N2 separation. They reported that the performance of polymeric membranes considerably improved upon the incorporation of magnetic particles due to the formation of magnetic channels as well as the interaction between particles and channels [[35], [36], [37], [38],[45], [46], [47], [48], [49], [50], [51]]. However, they did not investigate the effect of dispersion and arrangement of magnetic particles on the transport properties of magnetic MMMs. Therefore, it is worthy of investigating how and to what extent arrangement of particles can be employed to improve gas transport properties and separation performance.
The purpose of the present study is to improve the transport properties of PSf membranes for O2/N2 separation through incorporation of novel carbonyl iron powders (CIPs) into the membrane matrix at controlled conditions. CIPs were selected as the magnetic particles because of their superior magnetic properties enabling them to be arranged in the presence of magnetic fields. Moreover, CIPs possess high thermal stability as well as availability at a low prices [52,53]. The CIP/PSf magnetic MMMs were prepared in the presence of an external magnetic field in order to provide straight-through pathways for gas transport through the arrangement of CIP across the membrane thickness. Initially, the required magnetic field to arrange the particles was simulated by ANSYS Maxwell software. Then, the effect of different loading of CIPs on the gas transport properties of magnetically cast membranes was investigated in a newly devised magnetic gas separation module. The morphological structure, magnetic properties and gas separation properties of CIPs-PSf MMMs were investigated by SEM, VSM and gas permeation set-up. To the best of our knowledge, this is the first report on the application of CIPs in the field of gas separation membranes and entailing investigations on the effect of particles arrangement on the performance.
Section snippets
Materials
Polysulfone beads (PSf, Udel® P-3500 LCD MB7, Mw = 77,000–83,000 g/mol, density = 1.24 g/cm3) were purchased from Solvay Specialty Polymers and used as membrane matrix. Carbonyl iron powders (density=7.86 g/cm3) as magnetic particles were purchased from BASF group with the particle size range from 0.1–4.5 μm as determined based on SEM. 1-methyl-2-pyrrolidone (NMP, >99.5 %) was obtained from Merck and used as a solvent. Oxygen (O2, 99.999 %) and nitrogen (N2, 99.999 %) were purchased from
Effect of the external magnetic field on the morphology of MMMs
An optical microscope was used to observe the arrangement status of magnetic particles before and after applying the magnetic field. In order to study the effect of the application of an external horizontal magnetic field during membrane fabrication on the morphology of the membranes, three samples were prepared all containing 3 wt.% CIPs. Fig. 7 shows the optical microscopic images of the surface for the membrane prepared in the absence of any magnetic field (a), as well as those exposed to
Conclusions
In the present research, novel magnetic MMMs were prepared by incorporating CIPs into PSf matrix. An external magnetic field was used to create preferential magnetic channels for paramagnetic oxygen molecules in the cross section of membranes. The amount of magnetic force required to arrange the particles in the cross section of the membrane for the formation of magnetic channels was determined using Maxwell's software. The surface and cross-section SEM images as well as the gas transport
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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