Elsevier

Polymer Testing

Volume 91, November 2020, 106867
Polymer Testing

Biogas upgrading with novel cellulose nano-crystals and polyvinyl amine nanocomposite membranes

https://doi.org/10.1016/j.polymertesting.2020.106867Get rights and content

Highlights

  • Incorporation of CNC increased the facilitated transport of CO2.

  • Increasing CNC concentration increased membrane thickness and crystallinity.

  • Addition CNC does not affect the transportation of CH4.

  • Increasing feed pressure reduces membrane performance.

Abstract

A novel crystalline nano cellulose (CNC) and polyvinyl amine (PVAm) based nanocomposite membranes were synthesized and evaluated for biogas upgrading. Different concentrations of CNC was incorporated in 3 wt % PVAm solution on commercial polysulfone (PSf) sheet using dip coating method. The effect of feed pressure (5, 10 and 15 bar) was investigated for the CO2/CH4 separation. The incorporation of CNC increased the crystallinity of membranes. The thickness of selective layer enhanced to 2.16 μm from 1.5 μm with increasing concentration of CNC. However, degree of swelling reduced from 75.88% to 68.93 with CNC concentration at 1.5 wt%. The best results were shown by PVAm membrane with 1 wt % CNC concentration i.e. CO2 permeance of 0.0216 m3(STP)/m2.bar.hr and selectivity (CO2/CH4) of 41.The permeance decreased approximately 1.8 folds for PVAm/1CNC membrane with the increase in pressure from 5 to 15 bar. However, selectivity dropped from 41 to 39 for formulated membranes.

Introduction

Reduction in Greenhouse gas (GHG) emission is driving the global concerns due to its profound impacts on our climate. Carbon dioxide (CO2) a primary greenhouse gas, is mainly produced by burning of fuels and is estimated that the amount of CO2 will increased to 37 Gt by 2035 [1]. Currently, 80% of world's energy demands are fulfilled by fossil fuels. If we follow the same trend, fossil fuels reservoirs will be consumed in approximately the next 100 years [2]. Furthermore, in order to limit global warming, it is required that energy use would have to be totally decarbonizes and renewable must provide 65% of global energy demands by 2050 [3]. Therefore, it is necessary to find the sustainable and renewable energy resources with low carbon emission. Currently, 18.6% of total world's energy demands are being fulfilled by renewable energy sources. However, bioenergy accounts for approximately 14% [4]. Hence, bioenergy would be expected to the most potential and sustainable source of renewable energy for future global primary energy mix in 2050 [5].

Biogas is a form of bioenergy and product of Anaerobic Digestion (decomposition in an oxygen deficient environment) of organic waste. It is mainly comprising of CH4 (50–70) % and CO2 (30–50) %. The relative concentration of these two gases largely depends on nature of raw material and pH of bioreactor [6]. Biogas is being used for heating, production of steam and generation of electricity. However, after improving energy contents it can also be used as fuel for vehicles and grid stations. Presences of CO2, mainly reduces the calorific value of biogas and limit its utilization. Upgraded biogas is also called as bio methane (>95% CH4 contents) and can meet the technical requirements to replace the natural gas. Furthermore, the bio methane fuel has potential to reduce the non-methane volatile organic compounds emission by 50% and NOX emissions by 25%. Furthermore, a significant reduction in particulate emission [7].

Biogas is upgraded by various techniques such as; water or amine scrubbing, pressure swing adsorption, membrane technology, and absorption [2]. The membrane separation is a proven green technology with cost effective CO2 capture solution, and reduced footprints. Membrane technology has also been proven beneficial for low gas volumes and high CO2 contents [8]. Therefore, membrane technology is highly recommended for the biogas upgrading by CO2 removal [9].Various strategies have been employed in past to manufacture polymeric membrane with high efficiency, cost effectiveness and ease of fabrication. However, due to transport mechanism mainly based molecular sieving and kinetic diameter, the inherent trade-off between selectivity and permeability of polymeric membranes is a challenge. Therefore, to overcome this limitation, Facilitated Transport Membranes (FTM) were first introduced as Supported Liquid Membranes (SLM). The moveable carriers react with dissolved CO2. This complex is then transported across the membrane by solution diffusion mechanism. However, leakage of carrier in permeate and loss of solution by evaporation reduced the membrane performance with time. To overcome this problem, a new class of membranes has been evolved known as Fixed Site Carrier (FSC) membranes [10,11]. In FSC, the carrier is covalently bounded to the main polymer matrix. However, it reduced the free mobility of carrier but enhanced the overall stability and performance of membranes. Recently, research has been focused to make membrane material more hydrophilic to take the advantage of liquid membranes in highly swollen conditions [[12], [13], [14], [15]]. The FTM that works under highly swollen conditions facilitate the CO2 transport as bicarbonate ion (HCO3) through the membrane [16]. Utilization of such membranes have been reported in the literature and is revealed from the results that the degree of swelling is directly related to membrane performance [15,17,18]. Furthermore, number of nano filler and carrier molecules has been incorporated to enhanced swelling as well as affinity of composite membranes for CO2 [13,19].

Among different polymers used for acid gas separation, the polyvinyl amine is the most promising one. Due to the presence of abundant amine group and high degree of hydrophilicity, it gives high permeability and selectivity for CO2. Furthermore, PVAm is easily soluble in the water at room temperature [10,20]. PVAm has been extensively investigated for CO2 separation applications alone or with different combinations of fillers in mixed matrix membranes [10,12,21,22]. The structure of PVAm consists of amine group in its chain (-NH2) which has a natural affinity for CO2. It acts as fixed site carriers and facilitate transport of CO2 across the membrane. Recently, Zhao et al. has used PVAm in mixed matrix composite membranes with PANI/PS and results showed that the presence of PANI nanoparticles in PVAm matrix enhanced separation performance of composite membranes [23]. Further, Ming Wang and Zhi Wang et al. incorporated inorganic fillers such as MWCNT, SiO2 and ZSM-5 and study their interfacial properties. The study suggested that addition of nanofillers to PVAm matrix is an effective way to improve interfacial properties. However, better results could be obtained if inorganic filler and polymer has same functional groups [22]. But, stability issues of PVAm particularly at high pressure can be overcome by using high molecular weight PVAm or by introducing finely dispersed second phase in polymer matrix with high mechanical strength.

The Crystalline Nano cellulose (CNC) has been used in this research due to its high affinity with water and reinforcing nature [13,[24], [25], [26], [27]]. Cellulose fibers have hierarchical microstructures and on acid hydrolysis give nanostructure of highly crystalline regions (CNC) and amorphous regions (CNF) [13,[28], [29], [30]]. d-glucopyranose (C6H11O5) is a major component associated by β (1,4) links which is the repeating unit of cellulose [31]. The degree of polymerization of cellulose is difficult to determine but is reported to be near 10,000 if the molecular weight is around 3.2 × 106 g/mol. CNCs have become the center of attention for researchers due to its unique properties that include outstanding mechanical attributes, reinforcing capabilities, low density, biodegradability and excellent surface area per unit mass [13,18,24,28]. Moreover, cellulose is being abundantly used in bio and nanocomposites. Due to biodegradable nature, cellulose has replaced multiple synthetic fibers which also contribute in polluting environment. Cellulose is also being used as nonstructural biocomposite in doors, windows, ceiling tiles etc. [32]. Recently, CNCs have been reported as an additive with PVA and showed enhanced results of CO2 separation up to 15 bars. Furthermore, NFC has also been reported for enhanced performance of composite membranes. CNCs disperse along the polymeric matrix and help in moisture uptake and promotes swelling. This moisture content helps to increase the rate of facilitated transport of CO2 across the membrane [18]. Furthermore, PVA has also been chemically cross-linked with CNCs resulting in excellent thermal stability and reinforcement capability. Cross-linked PVA/CNCs have also been utilized in biocompatible electronic skin sensor system [33,34].

Thermodynamic properties of polymers play a vital role in the separation performance of membranes. Flory-Huggins theory describes the thermodynamics of polymer solutions and blends. It is a lattice model that explains the non-ideality of polymer mixtures. Comprehensive thermodynamic studies of polymeric blends have been carried out by Rana et al. for example, polyvinyl esters and polyacrylates, polystyrene-co-acrylonitrile and polyphenyl acrylate etc. Hydrogenated polymers were used as analogues of respective polymers and interaction energy densities were calculated [[35], [36], [37]]. However, in this work, polysulfone (PSf) and PVAm does not form blend and are chemically inert. Therefore, thermodynamics of these polymers have not been covered in this work.

This research work is carried out to improve the mechanical properties and water affinity of PVAm membranes to enhance CO2 separation at moderately high pressures. CNC has been incorporated in PVAm matrix in order to get beneficial results. There has been no chemical crosslinking between PVAm and CNC. The membranes were investigated for the optimized concentration of CNC in 3% PVAm solution. Effect of addition of different concentrations of CNC on degree of swelling was investigated. The SEM analysis of membranes was conducted to find the effect of incorporation of CNC on morphology and thickness of selective layer. The effect on degree of crystallinity of PVAm/CNC nanocomposite membranes was investigated using XRD. The membrane rig used for CO2 permeation testing was specially designed and has ability to work under humid conditions at moderately high pressure. Membranes were investigated under highly swollen conditions at 5, 10 and 15 bars. The results will be interpreted in terms of permeance of CO2 and CH4 and selectivity of CO2/CH4.

Section snippets

Materials

Ultrafiltration flat sheet membrane of Polysulfone (PSf) (Molecular weight cut-off 50,000) of the commercial grade was purchased from Alfa Laval. CNCs were acquired from Cellulose Lab, Canada. The average width and length of CNC was 12 nm and 170 nm, respectively. Polyvinyl amine (MW 17,000–20,000) was purchased from Sigma Scientific. The solvent used for casting of membranes was deionized water.

Preparation of composite membrane

PVAm was added to deionized water and stirred for 3 h to get 3 wt % solution of polyvinyl amine. The

Morphology and thickness of composite membranes

The scanning electron microscopy (SEM) analysis was performed to investigate the morphology and thickness of selective layer. SEM results revealed the smooth and defect free surface of PVAm/1CNC membrane as shown in Fig. 3(a). Furthermore, no agglomeration of nano particles was observed on the membrane surface. This indicates the even dispersion of nano particles within the polymeric matrix. Moreover, no cracks are visible on the membrane surface. Hence, the surface morphology does not show any

Conclusions

PVAm/CNC membranes were successfully fabricated and tested for the effect of CNC concentration and feed pressure on separation performance of CO2 and CH4. Addition of CNC has improved the permeance and selectivity of PVAm membrane for CO2 transport. However, addition of CNC does not showed any significant change on CH4 permeance. As compared to pure PVAm membrane, enhanced separation performance was observed when CNC was added in membranes. Furthermore, addition of CNC enhanced the moisture

Author Statement

Farooq sher and Zaib Jahan conceptualized the project. Uzair Saeed and M. Bilal Khan Niazi performed investigation and formal analysis. Erum Pervaiz did the project administration. Uzair Saeed, Zaib Jahan and M. Bilal Khan Niazi wrote the original draft.

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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