Synthesis and performances of bio-sourced nanostructured carbon membranes elaborated by hydrothermal conversion of beer industry wastes
© El Korhani et al.; licensee Springer. 2013
Received: 31 October 2012
Accepted: 18 February 2013
Published: 7 March 2013
Hydrothermal carbonization (HTC) process of beer wastes (Almaza Brewery) yields a biochar and homogeneous carbon-based nanoparticles (NPs). The NPs have been used to prepare carbon membrane on commercial alumina support. Water filtration experiments evidenced the quasi-dense behavior of the membrane with no measurable water flux below an applied nitrogen pressure of 6 bar. Gas permeation tests were conducted and gave remarkable results, namely (1) the existence of a limit temperature of utilization of the membrane, which was below 100°C in our experimental conditions, (2) an evolution of the microstructure of the carbon membrane with the operating temperature that yielded to improved performances in gas separation, (3) the temperature-dependent gas permeance should follow a Knudsen diffusion mechanism, and (4) He permeance was increasing with the applied pressure, whereas N2 and CO2 permeances remained stable in the same conditions. These results yielded an enhancement of both the He/N2 and He/CO2 permselectivities with the applied pressure. These promising results made biomass-sourced HTC-processed carbon membranes encouraging candidates as ultralow-cost and sustainable membranes for gas separation applications.
In the recent years, attention has been focused on carbon-based nanomaterials to face environmental issues . Mainly in the form of carbon nanotubes, these nanomaterials were advantageously used as building blocks for water filtration and gas permeation membranes, adsorbents, and environmentally friendly energy applications such as gas storage or electrodes for (bio) fuel cells [2–8]. Since 1980, carbon membranes have shown interesting performances, particularly in gas separation . The chemical and physical features of carbon nanomaterials experimentally depend on the raw materials and on the preparation process. In a global and integrated sustainable route, biomass can be advantageously used as a carbon source [2, 5, 10–18]. These raw materials obviously represent a very low-cost alternative and possible high availability compared to petrol-derived commercial carbon sources such as classical polymeric precursors: polyacrylonitrile, polyamide and its derivatives, and cellulose [9, 16]. In order to convert biomass (carbohydrate) into carbon materials, two main routes can be used, namely a pyrolysis approach or a hydrothermal carbonization (HTC) [10–12, 19, 20]. The HTC process can provide carbon materials with low energy consumption (<350°C) and with limited environmental impacts due to the non-generation of CO2 during conversion reactions. The HTC process is usually performed in a sealed autoclave and in the presence of water [10–12]. In 1913, Bergius has done pioneer works on cellulose conversion to carbon materials. The process he developed was thus extended to various carbon sources like carbohydrates such as glucose [10, 12, 21]. In a similar field, Antonietti et al. have performed pioneer works by elaborating a variety of carbon-based microstructures and nanostructures from hard or soft sources such as orange peels, oak leaves, pine cones, pine needles, and rice [10–12, 17, 22, 23].
In the present study, our aim was to produce ultralow-cost membranes by sustainable routes to answer environmental issues (water and air filtrations) affecting some emerging and third countries, such as Lebanon. The strategy we developed is based on the valorization of natural products and food industry by-products. We develop a process based on the hydrothermal carbonization of Lebanese beer wastes to produce carbon-based nanoparticles (NPs). The obtained NPs were then used to produce carbon membranes of which performances in water filtration and gas separation will be presented and discussed.
Synthesis of carbon-based nanoparticles by hydrothermal carbonization
Carbon nanoparticles were synthesized from beer wastes by a hydrothermal carbonization process. Beer wastes were obtained from Almaza Brewery (Heineken International, Beirut, Lebanon), rated as the first brewery in Lebanon since 1933. The wastes were collected after the filtration process of beer mixture and are essentially composed of malt, water, and yeast. After drying at 100°C for 14 h, the ensuing solid was ground in a ball miller for 4 h at 200 rpm. Citric acid (Sigma-Aldrich Co., Dorset, England, UK) was used as an activating agent in the carbonization reaction .
The reaction was carried out in a non-stirred, 300-mL capacity Teflon-lined stainless steel autoclave (Parr Instrument Company, Moline, Illinois, USA), in which the temperature is controlled by a thermocouple (Eurotherm regulator, Invensys Eurotherm, Ashburn, VI, USA). During heating and due to experimental setup limitation, the temperature cannot exceed 350°C and the pressure of 200 bar. In a typical experiment, 15 g of beer wastes was dispersed in 120 mL of pure water for 30 min, and then, 30 mg of citric acid was added as an activating agent for the carbonization reactions occurring during the HTC process. The autoclave was sealed and heated up to 220°C for 16 h, these operating conditions being similar to HTC biomass conversion . After the HTC process, the crude product contained a precipitate, the biochar (or hydrochar) and a colloidal solution, which were easily separated by centrifugation (8,000 rpm; 30 min). The charcoal was washed several times with water (18 mΩ) and then dried in an oven at 80°C for 12 h before further characterization. The colloidal solution was used as obtained. Alternatively, the colloids were destabilized by the addition of ammonia solution (1 M) up to pH of approximately 9 to yield the formation of a precipitate by colloid aggregation. The solid was filtered off on a 0.45-μm micrometric filter (Whatman, Maidstone, UK) and washed several times with water (18 mΩ), and then dried in an oven at 80°C for 2 h. For experimentation of the HTC process, it is necessary to keep in mind that for security reasons, starting solution should not exceed two thirds of the total autoclave volume.
Carbon membrane preparation
In order to prepare porous carbon membranes, the obtained colloidal solution was concentrated at 70% (v/v) and then deposited on the inner surface of low-ultrafiltration tubular alumina membranes (ES 1426, 5 nm, Pall Membralox, NY, USA) by slip casting. The obtained membrane was treated in a tubular oven under a nitrogen atmosphere up to 1,000°C (120°C/h up to 500°C (1-h dwell) then 180°C/h up to 1,000°C (3-h dwell)).
The soluble fraction of the used beer waste was characterized by 1H nuclear magnetic resonance (NMR) using a Bruker Advance 300-MHz NMR spectrometer (Madison, WI, USA), using D2O as reference solvent. Infrared (IR) spectroscopy was performed using the KBr pellet technique using a Spectrum Nicolet 710 Fourier-transformed IR (FTIR) apparatus in the range 4,000 to 450 cm-1. Raman spectrum was recorded at room temperature with an Ar-Kr laser LabRAM 1B spectrometer (HORIBA, Ltd., Kyoto, Japan). The morphology of carbon particles was observed by scanning electron microscopy (SEM) using a HITACHI S4800 microscope (Chiyoda-ku, Japan). Transmission electron microscopy (TEM) was used for deep investigation of the nanoparticles produced. This was carried out using a Philips CM20 microscope (Amsterdam, The Netherlands) operating at 200 KV at a resolution of 1.4 Å. Carbon membranes were analyzed by nitrogen adsorption-desorption isotherm using BET techniques (ASAP 2012, Micromeritics, Norcross, GA, USA) in order to identify the specific surface area of the membrane and estimate the pore diameters. Scanning electron microscopy was also used to visualize the morphology and the thickness of the elaborated carbon layer. The water filtration experiments were conducted on a home-made filtration pilot. The dynamic gas permeation test was performed on a classical separation pilot using N2, He, and CO2.
Results and discussion
Beer waste characterization
Carbon nanoparticles preparation and characterization
A suspension of beer wastes particles in aqueous citric acid was used as starting solution for the hydrothermal carbonization process. After reaction, the solid charcoal was separated from a colloidal solution by centrifugation. For analysis purposes, the carbon-based nanoparticles were precipitated upon aggregation by addition of ammonia solution (1 M) up to pH of approximately 9.
Morphological characterization of the nanoparticles
Carbon membrane preparation and characterization
As illustrated in Figure 11b, no water flux was measured with carbon membranes below 6 bar of applied nitrogen pressure. The measured permeability is 0.005 L h-1·m-2·bar-1, a value which is 1,000 lower than the commercial alumina system. This result can be interpreted as a very low porous volume accessible inside the membrane. Therefore, this membrane cannot be used for water filtration applications. The quasi-dense behavior of the carbon membrane for low applied external pressure emboldens us to evaluate this material for gas separation.
Hydrothermal carbonization process of beer wastes (Almaza Brewery) yields a biochar and homogeneous carbon-based nanoparticles (NPs). Carbohydrates, released by the wastes in water, are supposed to play a role in the formation mechanism of the NPs, and further experiments will be driven in the future to elucidate the latter. The NPs have been used to prepare carbon membrane on commercial alumina support. As evidenced in water filtration experiments, there is a quasi-dense behavior of the membrane with no measurable water flux below an applied pressure of 6 bar. Gas permeation tests were conducted and gave remarkable results: (1) the existence of a limit temperature of utilization of the membrane is below 100°C in our experimental conditions; (2) an evolution of the microstructure of the carbon membrane with the operating temperature yielded to improvement in its gas separation performances; (3) the permeance of the gas is temperature dependent and should be driven by a Knudsen diffusion mechanism; and (4) the He permeance is increasing with the applied pressure in entrance on the system, whereas N2 and C02 permeances are stabilizing in the same conditions. This result yields an increase of the selectivity He/N2 and He/CO2 with the applied pressure. The obtained selectivity values are below the ones reported in the literature but further experiments are in progress in order to improve this value by optimizing the membrane microstructure and porosity. These promising results made biomass-sourced HTC-processed carbon membranes promising candidates as ultralow-cost and sustainable membranes for gas separation applications. Since He exhibits a kinetic diameter closed to that of H2, applications as membrane for H2 separation can be envisaged, for instance, for fuel cell applications.
The authors would like to thank the Lebanese ALMAZA company for providing large quantities of beer wastes, especially Maroun Ammouri, and would also deeply thank Christophe Charmette (IEMM) and Gilles Nunez for conducting the gas separation measurements. Authors are grateful to Eddy Petit, Didier Cot, and Abed-el-Salam Mansouri for their cooperation in the membrane characterizations. Thanks to the Erasmus Mundus EC JOSYLEEN program for the Ph.D. grant.
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