Development of membrane processes for the selective separation of CO2 from biogas and biohydrogen

Abstract

Despite the boom in renewable energies occurred in recent years, the demand for fossil fuels, a non-renewable energy source, continues to grow steadily. Indeed, fossil fuels currently accounts for 80% of global energy demand, which entails high greenhouse gas (GHG) emissions. In order to reduce the use of fossil fuels and GHG emissions, regulations have been introduced to promote the creation of a new energy system to replace the so-called "dirty fuels" with renewable energies, with the ultimate goal of mitigating climate change. In this context, biogas and biohydrogen, due to their renewable nature and zero carbon footprint during combustion, are considered two of the gaseous biofuels that will replace current fossil fuels. Biogas from the anaerobic degradation of organic waste, composed mainly of CH4 (45-85%) and CO2 (25-50%), could be used as an energy feedstock to export energy to the electricity grid or central heating networks of cities. On the other hand, biohydrogen is composed mainly of H2 and CO2, whose composition is determined by the type of generation process, with contents of 40-60% of H2 and 47-60% of CO2 in bioreactors for dark fermentation of organic waste. Biogas must be purified and converted into high quality biomethane meeting current regulatory standards prior use as a vehicle fuel or injection into natural gas networks. Likewise, the enrichment of biohydrogen requires the removal of CO2, which is the main pollutant of this gaseous biofuel. Currently, the removal of CO2 from both biogas and hydrogen generated from natural gas on an industrial scale is carried out by means of physical/chemical technologies, which suffer from high operating costs and corrosion problems. Biological technologies for CO2 removal from biogas, such as photosynthetic enrichment and hydrogenotrophic enrichment, are still in the experimental development phase. Among the technologies currently implemented on an industrial scale for the removal of CO2 from biogas, membrane separation is the only one with the potential to improve the performance of CO2 separation from both biogas and biohydrogen, and to reduce investment and operating costs, as a result of the current advances in the field of nanotechnology and materials science. Membrane purification of biogas and biohydrogen is based on the principle of selective permeation through semi-permeable membranes. Among the broad range of membranes for gas separation, polymeric membranes have proved to be very competitive, and dominate the market due to a combination of good processability, low price, ease of scale-up and ability to adjust their composition to the target application. However, full industrial-scale exploitation of these membranes has not yet been achieved. The main challenges in the field of membrane technology are a) obtaining good selectivity without sacrificing permeability and b) maintaining long-term gas separation performance by limiting physical aging and plasticization. In order to overcome these challenges, advanced materials such as Polymers of Intrinsic Microporosity (PIMs), Thermally Rearranged (TR) polymers based on polybenzoxazoles (PBO) and organic and inorganic fillers (particularly of submicron and nanometer size) incorporated into a polymeric matrix to manufacture Mixed Matrix Membranes (MMMs) with enhanced membrane selectivity and higher capacity to exceed the selectivity-permeability upper limits established by Robeson in his 1991 and 2008 publications in the Journal of Membrane Science. Since polymeric membranes have restrictions due to the trade-off between gas permeability and selectivity (high permeability, low selectivity and vice versa), this PhD thesis focused on the development of a new membrane technology for gas separation capable of overcoming the challenges of conventional polymeric membranes, generating new membranes with improved separation properties. The manufacture of polymeric membranes such as MMMs and thermal rearrangement modified MMMs (TR-MMMs) could lead to higher permeability and selectivity in CO2 separation during biogas and biohydrogen upgrading. In this PhD thesis, MMMs were initially fabricated, using PPNs as organic fillers and polyamides as polymeric matrices for the separation of CO2 from biogas and biohydrogen, capable of overcoming the inherent trade-off between permeability and selectivity. In particular, the exceptional thermal properties of PPNs were combined with thermally rearrangeable polyamides to investigate the transport properties in TR-MMMs. First, a systematic evaluation of the CO2 and CH4 gas permeability properties of these MMMs and TR-MMMs was carried out. For this purpose, a hydroxy-polyamide (HPA, 6FCl-APAF) with high molecular weight was synthesized and used as a polymeric matrix for the manufacture of MMMs using PPN-2 (trypticene- trifluoroacetophenone) as filler at 10, 15, 20, 30 and 40% (on weight basis). Additionally, the prepared MMMs were subjected to heat treatment to induce HPA rearrangement to a polybenzoxazole (ß-TR-PBO) and obtain the corresponding TR-MMMs. To fully characterize the polymers, the combination of information from multiple physicochemical techniques was required to elucidate the structure of the investigated materials and to establish the limits of thermal stability, structure, and conditions for further processing of the material. In this study, membrane permeability for pure gases (N2, O2, CH4 and CO2) was measured at 35 °C and 3 bar in a constant-volume pressure-variable permeation apparatus, obtaining with these measurements the ideal selectivity for the gas pairs CO2/CH4, CO2/N2 and O2/N2. An increase in the permeability of the MMMs with respect to the polymer matrix was observed for all gases as the PPN content increased. The improvement in the permeability of TR-MMMs (ß-TR-PBO based on HPA) was considerably greater than the increase in MMMs. In the particular case of membranes loaded with 30% PPN, the permeabilities to CO2 and CH4, both before and after thermal rearrangement, increased to reach the 2008 Robeson upper bound. These relatively good permeabilities were attributed (among other causes) to good particle-matrix interaction, generating membranes free of interfacial voids. However, this increase in permeability was not observed for MMMs loaded with 40% PPN. The decrease in permeability at 40% was attributed to the formation of interfacial voids generated by particle agglomeration. The CO2/CH4 selectivity of MMMs, as well as TR-MMMs, decreased slightly both when PPN loading was increased and when thermal rearrangement occurred. Based on the results obtained in the first part of this PhD Thesis, the optimum loading for this polymeric matrix, 6FCl-APAF, was set at 30% of PPNs. The potential of these membranes for H2 separation was evaluated by determining the permselectivity of hydrogen to CH4, CO2 and N2 in the MMMs and their corresponding TR-MMMs, under the same operating conditions. A remarkable increase in permeability and selectivity properties was observed for H2/N2 and He/CH4 gas pairs with 30% loaded membranes compared to polymer matrix and 20% loaded membranes. The same behavior was observed in the TR-MMMs, although with a slight decrease in selectivity. Although the expected results were not obtained for the H2/CO2 gas pair (of major importance in this research work), leading to a selectivity close to 1 for the case of TR-MMM membranes, a remarkable improvement could be appreciated in comparison with conventional membranes for H2 separation not subjected to thermal rearrangement. The unique gas transport behavior of the evaluated membranes demonstrates the advantages of PPN as gas separation membrane filler for the potential separation and purification of H2 and CO2 capture. Finally, the synthesis and testing of new materials, containing m-Terphenyl groups, used as polymeric matrices in the manufacture of MMMs, was carried out. A hydroxypolyamide (HPA), a polyamide (PA) and a copolymer (Co-HPA-PA), produced by stoichiometric copolymerization of the diamines APAF and 6FpDA and using 5'-tert-butyl-m-terphenyl-4,4''-dicarboxylic acid chloride (tBTpCl) for all syntheses performed were synthesized. The MMMs were loaded to 20% of PPNs, using the previously used PPN-2 and a new filler material PPN-1, formed from isatin and trypticene. The membranes obtained were subjected to heat treatment to convert HPA to polybenzoxazole to obtain TR-MMMs and to evaluate the gas separation properties for H2, N2, O2, CH4 and CO2 at 35 °C and 3 bar, following the same operating conditions as in the previous work. Compared to the polymeric matrices, both thermal rearrangement and the addition of PPNs increased permeability with slight decreases in selectivity for all gases tested. The trade-off of permeability versus selectivity yielded excellent results, especially for the H2/CH4 and H2/N2 gas pair, approaching the 2008 Robeson upper bound. A comparison of transport properties using PPN-1 and PPN-2 was also carried out, observing that the best gas separation properties were obtained with the use of PPN-2 as filler, which was attributed to a better polymer-filler interaction. Characterization of gas permeability as a function of intersegmental chain distance, the latter obtained by wide-angle X-ray diffraction (WAXD), and membrane free volume fraction together with kinetic diameters of permeated gases, was carried out. It was observed that after the thermal rearrangement process and by introducing PPNs into the polyamide backbone chain, the d-spacing was increased, leading to an increase in gas permeability due to the increase in intersegmental distance. It was also observed that the selectivity was not noticeably affected probably due to the increased stiffness of the chains. In terms of free volume, permeability followed an exponential trend versus free volume and the quadratic function of the kinetic gas diameter. The combination of a polyamide with high H2 permeability and PPNs to confer high H2/CH4 and H2/N2 selectivity to the material, resulted in composite membranes that overcome the performance limitation of pure polymeric membranes. In this thesis work, it was shown that, as in the case of MMMs, subtle changes in polymer chemistry as well as fillers can have important effects on the polymer-filler interactions and properties of TR-MMMs, both before and after thermal rearrangement. It was further demonstrated that advanced nanoporous materials currently being explored as fillers in MMMs can also be applied to improve the gas separation performance of polymers suitable for producing benzoxazoles, following the same strategy in MMMs.