Natural gas (NG) is the one of cleanest, safest, and most efficient fossil fuel, which emits roughly 26–41% less CO2 than oil and coal (Adewole et al. 2013). Hence, the demands on NG keep growing with an expected 64% more consumption in the coming few decades. To meet the expanding global demand for NG, offshore NG fields offers an alternative reserves to onshore counterparts and the ocean provides about one-third of global NG (Chen et al. 2017). For offshore NG production, a complete production process is carried out on-site of the floating liquefied NG production storage and off-loading (LNG-FPSO) structures, including pre-treatment, liquefaction, storage, transportation, and off-loading (Chen et al. 2020). The pre-treatment is the most crucial section of the offshore NG development to remove impurities from raw NG before commercializing it. Raw NG comes with high compositions of methane while CO2 and H2S are the major impurities (Adewole et al. 2013). Raw NG contains up to 70% of CO2 depending on the geographical location, and the removal procedure is typically performed at a pressure range of 30–60 bar (Han and Ho 2021). Such high amounts of CO2 in the NG stream reduce its energy content (calorific values) and create corrosion problems during transportation in pipelines and cylinders. Thus, CO2 content should be minimized to 2–3% by volume (Lee et al. 2018).
To date, the leading technologies for CO2 removal from NG are solvent scrubbing, adsorption, membranes separation, and cryogenic distillation. Among all, amine-scrubbing in a gas-liquid contactor is the most mature and commercially used method for CO2 removal, that offers higher capture efficiency (Mukhtar et al. 2020c). However, this process is high energy demanding, high operational cost, and operates in central plants making it not suitable for offshore NG treatment (Adewole et al. 2013; Chen et al. 2020). Adsorption technology offers alternative simplified operation, low energy demand, ease of control, and high efficiency in addition to wide range of adsorbent materials (Chen et al. 2020). Broadly, different types of solid adsorbents have been explored for CO2 capture from NG, including carbon-based materials (Attia et al. 2020), zeolites (Wang et al. 2019), mesoporous silica (Ullah et al. 2015), microporous organic polymer (MOPs) (Xu et al. 2020) and metal-organic framework (MOFs) (Furukawa et al. 2010). All of such adsorbents are packed in fixed bed column for practical applications and this is accompanied by pressure loss, channelling, and inhomogeneous gas flow when fitted for working in the adsorption columns processes such as pressure or temperature swing adsorption (Mallick et al. 2018). Moreover, MOFs bearing extraordinary CO2 adsorption capacity have high cost, low hydrothermal stability and are not practical for column system at a large scale of LNG-FPSO (Abid et al. 2012; Danaci et al. 2015).
Solid basic polymer adsorbents containing groups such as hydroxy, nitro, amine, imidazole, triazine, and imine provide alternative materials for CO2 capture (Petrovic et al. 2021). Particularly, the development of new adsorbents with amine-containing fibrous structures provides advantages in terms of rapid gas diffusion and enhanced gas-solid interaction while reducing pressure loss during gas treatment. Graft copolymerization is one of the most appealing methods to impart permanent functional groups to polymer substrates (non-woven fabrics, films, and porous particles) to prepare CO2 polymer adsorbents with various morphologies (Zhao et al. 2020). Graft copolymerization can be initiated on polymer substrates using high energy radiation including gamma rays, electron beam, low energy radiation such as ultraviolet (UV), and plasma treatment in addition to conventional chemical initiators in the presence of vinyl monomers that can host different amine groups (Shoushtari et al. 2012).
Fibrous amine containing adsorbents is a class of solid polymer adsorbents that have potential to overcome the pressure drop and gas channelling problems when packed in adsorption columns and thus they are worthy further development. Moreover, most of these aminated fibrous adsorbents were tested for CO2 capture from air and post-combustion effluent despite the presence of other major applications involving NG purification. Fibrous adsorbents were mainly prepared by radiation induced graft copolymerization (RIGC) of vinyl monomers onto polymer non-woven sheets made of polypropylene and polyethylene/polypropylene (PE/PP) (Nasef et al. 2014; Kavaklı et al. 2016; Rojek et al. 2017; Abbasi et al. 2018). Monomers such as glycidyl methacrylate (GMA) (Imanian et al. 2022), N-vinylformamide (Zubair et al. 2020), and acrylamide (Nasef and Güven 2012) were used to endow variety of amine groups after post-grafting treatments and demonstrated an appealing affinity to CO2 (Nasef and Güven 2012). Applying RIGC for immobilization of functional groups offers advantages in terms of ability to control the level, location, and distribution of graft chains on the substrate by optimization of reaction parameters without leaving detrimental waste. This allows the adsorbent to be easily scaled and makes the preparation process rather environmentally friendly (Abou Taleb et al. 2008).
Several studies have reported the preparation of fibrous adsorbents by RIGC of GMA, which have an epoxy group allowing the amination of the grafted substrate with various amine groups (Abbasi et al. 2018, 2019b; Mohamad et al. 2021). However, the investigated fibrous CO2 adsorbent have been mainly tested for adsorption of CO2 from its mixtures with N2 whereas their application in CO2 capture from its mixtures with CH4 have not been reported in the literature. Moreover, the adsorption isotherms, kinetics, and thermodynamics of adsorption on such aminated fibrous adsorbents have not been established. The objective of the present study is to investigate the CO2 adsorption behaviour from CO2/CH4 mixtures on fibrous adsorbent immobilized with various amine groups hosted by poly (GMA) incorporated in PE/PP fibrous sheet by RIGC. The various properties of the adsorbents were evaluated. The performance was evaluated under various temperatures and pressures with different CO2/CH4 mixtures resembling the conditions of industrial removal of CO2 from NG. Moreover, the adsorption isothermal, kinetic, and thermodynamic behaviours were studied by fitting the data to common models. The obtained fundamental properties such as adsorption capacity, kinetics, thermodynamics, and selectivity are essential for laying the foundation for process design parameters for CO2 capture from NG with such fibrous adsorbents.