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Vacuum Swing Adsorption on Natural Zeolites from Tuffs in a Prototype Plant Francesco Petracchini,a Valerio Paolini,a Flavia Liotta,a Lucia Paciucci,a and Enrico Faccib aInstitute of Atmospheric Pollution Research, National Research Council of Italy, Monterotondo, Rome 00015, Italy; v.paolini@iia.cnr.it (for correspondence) bAzzeroCO2 s.r.l, Rome 00184, Italy Published online 00 Month 2017 in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/ep.12530 A vacuum swing adsorption (VSA) prototype plant was developed and tested, based on natural zeolites from tuffs containing Chabazite. The regeneration procedure of natural zeolites was optimized, and the operative pressure resulted to be the most important parameter. The VSA prototype was fed for 3 months with a real biogas, produced by agrozootechni- cal byproducts in an anaerobic baffled reactor (ABR). Biogas composition from ABR was very stable and allowed to deeply investigate the performances of natural zeolites in real bio- gas conditions. Biogas was desulfurized and dehydrated before the VSA unit. In our experimental conditions, a bio- methane average purity>98% was achieved, with methane recovery > 95%. CO2 concentration was always lower than 0.3%, while oxygen ranged from 0.2 to 1.5%. A light desul- furization effect was also observed. CH4 concentration was < 0.1% in the VSA off-gas. VC 2017 American Institute of Chemical Engineers Environ Prog, 00: 000–000, 2017 Keywords: biogas, zeolites, adsorption, methane, pressure swing adsorption INTRODUCTION Biogas from anaerobic digestion is a useful fuel suitable for the production of thermal and electrical energy [1–3]. The composition of biogas, mainly methane (40–75%) and carbon dioxide (15–60%), depends on the feeding and the opera- tional conditions of the anaerobic digester. Trace amounts of others molecules are also present, including water (5–10%), hydrogen sulfide (0.005–2%), siloxanes (<0.02%), halogenat- ed hydrocarbons (<0.6%), ammonia (<1%), oxygen (0–1%), carbon monoxide (<0.6%), and nitrogen (0–2%) [4]. Biogas treatments aim to remove trace components potentially harmful to the natural gas grid, appliances or end-users [5,6]. Biogas upgrading to biomethane is usually performed in order to improve the energy conversion effi- ciency, by removing CO2, and adjusting the calorific value. Biomethane is typically 95–97% CH4 and 1–3% CO2, and can be used in vehicles or injected in national natural gas grids. In spite of the high economical interest in biomethane tech- nology, at the current state of the art there is still a lack of harmonization in National technical standards (Table 1). Indeed, both in EU and in US there is not a communitarian/ federal standard for biomethane use, neither for grid injec- tion nor for use as vehicle fuel. Moreover, some standard requires minimum methane content, while many others VC 2017 American Institute of Chemical Engineers merely indicate Wobbe index ranges. As a consequence, the compliance with grid injection or autovehicle standards depends on the considered country. Currently, several upgrading technologies allow to pro- duce biomethane and remove carbon dioxide: physical and chemical CO2 adsorption, pressure swing adsorption (PSA) and vacuum swing adsorption (VSA), membrane separation, cryogenic separation, and biological methane enrichment [4,6]. VSA consists in removing CO2 with an adsorbent; once the adsorbent is saturated regeneration is performed by pres- sure reduction [7]. Compared with other technologies, VSA presents several advantages in terms of efficiency, biome- thane purity, scalability, and total costs: as a consequence, VSA has become a consolidated approach in biomethane technology [6,7]. Several adsorbents are known for CO2 adsorption, and the most used materials in industrial VSA applications are synthetic zeolites [8] and activated carbon [9,10]. However, in the last years the possibility to perform a PSA/VSA on natural and more economic adsorbents has also being investigated: for instance, sepiolite [11], silicalite [12], and mudrocks [13] gave interesting results. Among several promising adsorbents suitable for biogas upgrading, natural zeolites are a low cost material available in large amounts from the building industry, and they are usually composed of chabazite, phillipsite, and clinoptylolite [14,15]. Natural zeolites were investigated for the adsorption of CO2 and H2S in synthetic gas mixture [16–21]. Results obtained suggest that natural zeolites can be successfully used in biogas purifi- cation and upgrading plants, as a valid alternative to synthet- ic zeolites and activated carbon. A comparison of several adsorbents in terms of adsorption capacities is given in Table 2: even if a lower adsorption capacity was observed for natu- ral zeolites, an industrial application of these materials is still attractive, due to their lower cost. The efficiency of an adsorbent for biogas upgrading can- not be evaluated only by its adsorption capacity: indeed, moisture and secondary components of biogas can dramati- cally affect the adsorption performances of synthetic zeolites [22,23]. Nevertheless, variations in biogas composition can also hamper the upgrading efficiency [24]. In a previous study [21], natural zeolites were tested for the upgrading of real biogas streams, and vacuum regeneration was investigat- ed: the compatibility with moisture conditions of real biogas was demonstrated, and a good vacuum regenerability was observed. On the basis of these promising results, this Environmental Progress & Sustainable Energy (Vol.00, No.00) DOI 10.1002/ep Month 2017 1PDF Image | Vacuum swing adsorption on natural zeolites
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