Oxygen enrichment of air: Performance guidelines for membranes based on techno-economic assessment

The use of oxygen-enriched air (OEA) is an important strategy for reducing process cost and CO2 emissions in several industrial processes. Membrane-based processes, attributing to their high energy efficiency, are promising alternatives to cryogenic distillation and pressure-swing adsorption (PSA) at small to medium scales. However, so far, the techno-economic assessment of membrane processes has been focused on the production of low-purity O-2. This study assesses O-2 enrichment by membrane-based processes in a large purity range (30-95%), and reports significant energy and cost savings when high-performance membranes, marked by O-2 permeance of 500-1000 GPU and O-2/N-2 selectivity in the range of 2-20, are developed. The analysis identifies that O-2/N-2 selectivities of 2-4, 6-15, and 20 are optimal for target purity of 30-40%, 50-65%, and >70%, respectively, leading to specific costs in the range of 20-50 $/ton(EPO2) when blending with air is not considered. Purity target below 75% can be achieved by the single-stage process while double-stage processes are needed for higher purity targets. If highly-selective membranes are available, the cost of lower purity OEA can be further reduced by blending higher purity OEA with air. We demonstrate the benefits of the membrane process for four important applications. For natural gas combustion, optimal energy and cost savings of 41% and 22%, respectively, are predicted using membranes with selectivity of 4. An optimized hybrid system combining membrane enricher and post-combustion capture reduces energy consumption (22%) and cost (6%) with respect to the standalone capture. The production of medical-grade oxygen (90% purity) from membranes is economical (0.4$ /kL) compared to that from PSA (0.6-0.8 $/kL). Finally, 95% O-2 can also be produced for oxycombustion with a specific cost of 32$ /ton(EPO2).

Gas Separation Membranes with Atom-Thick Nanopores: The Potential of Nanoporous Single-Layer Graphene

Kumar Varoon Agrawal, Deepu Joseph Babu, Kuang-Jung Hsu, Cédric Karel J Van Goethem, Luis Francisco Villalobos Vazquez de la Parra

Gas separation is one of the most important industrial processes and is poised to take a larger role in the transition to renewable energy, e.g., carbon capture and hydrogen purification. Conventional gas separation processes involving cryogenic distillation, solvents, and sorbents are energy intensive, and as a result, the energy footprint of gas separations in the chemical industry is extraordinarily high. This has motivated fundamental research toward the development of novel materials for highperformance membranes to improve the energy efficiency of gas separation. These novel materials are expected to overcome the intrinsic limitations of the conventional membrane material, i.e., polymers, where a longstanding trade-off between the separation selectivity and the permeance has motivated research into nanoporous materials as the selective layer for the membranes. In this context, atom-thick materials such as nanoporous single-layer graphene constitute the ultimate limit for the selective layer. Gas transport from atom-thick nanopores is extremely fast, dependent primarily on the energy barrier that the gas molecule experiences in translocating the nanopore. Consequently, the difference in the energy barriers for two gas molecules determines the gas pair selectivity. In this Account, we summarize the development in the field of nanoporous single-layer graphene membranes for gas separation. We start by discussing the mechanism for gas transport across atom-thick nanopores, which then yields the crucial design elements needed to achieve high-performance membranes: (i) nanopores with an adequate electron-density gap to sieve the desired gas component (e.g., smaller than 0.289, 0.33, 0.346, 0.362, and 0.38 nm for H2, CO2, O2, N2, and CH4, respectively), (ii) narrow pore size distribution to limit the nonselective effusive transport from the tail end of the distribution, and (iii) high density of selective pores. We discuss and compare the state-of-the-art bottom-up and top-down routes for the synthesis of nanoporous graphene films. Mechanistic insights and parameters controlling the size, distribution, and density of nanopores are discussed. Fundamental insights are provided into the reaction of ozone with graphene, which has been successfully used by our group to develop membranes with record-high carbon capture performance. Postsynthetic modifications, which allow the tuning of the transport by (i) tailoring the relative contributions of adsorbed-phase and gas-phase transport, (ii) competitive adsorption, and (iii) molecular cutoff adjustment, are discussed. Finally, we discuss practical aspects that are crucial in successfully preparing practical membranes using atom-thick materials as the selective layer, allowing the eventual scale-up of these membranes. Crack-and tear-free preparation of membranes is discussed using the approach of mechanical reinforcement of graphene with nanoporous carbon and polymers, which led to the first reports of millimeter-and centimeter-scale gas-sieving membranes in the year 2018 and 2021, respectively. We conclude with insights and perspectives highlighting the key scientific and technological gaps that must be addressed in the future research. Superscript/Subscript Available

AMER CHEMICAL SOC2022

Thermo-chemical H2 production: Thermo-economic modeling and process integration

François Maréchal, Laurence Tock

Within the global challenge of climate change and energy security, hydrogen is considered as a promising decarbonized energy vector to be used in electricity production and transportation. In this paper, the thermo-chemical production of hydrogen by natural gas reforming and by lignocellulosic biomass gasification are analyzed, compared and optimized by developing thermo-economic models. Combining flowsheeting with process integration techniques, thermo-economic analysis and life cycle assessment (LCA), a systematic comparison of different process options with regard to energy, economic and environmental considerations is made. The choice of the technologies is optimized together with the operating conditions using multi-objective optimization. In both natural gas and biomass based H2 pathways, a CO2 removal step is included during the H2 purification which allows for CO2 capture and further sequestration. The potential for greenhouse gas mitigation is assessed and compared with conventional plants without capture based on the CO2 avoidance cost and the overall CO2 equivalent emissions computed from the life cycle chain. The system’s performance is improved by introducing process integration valorizing the waste heat by the combined production of heat and power. The H2 application purpose and the corresponding required purity are key factors defining the process performance. The trade-offs between competing thermoenvironomic (i.e. energy, economic and environmental) objectives are finally assessed using a multi-objective optimization. For natural gas based H2 production overall energy efficiencies up to 80% and production cost of 22-110 USD/MWH2 are computed compared to around 60% efficiency and 75-263 USD/MWhH2 for biomass based processes having the advantage of using renewable resources. The CO2eq emissions are reduced by more than 6.4kgCO2eq/kgH2 for NG and 20kgCO2eq/kgH2 for BM processes compared to the cases without CO2 capture. The competitiveness on the energy market depends strongly on the resource price and on the imposed CO2 taxes. Our study shows that the thermo-chemical hydrogen production has to be analyzed as a polygeneration unit producing not only hydrogen but also captured CO2 and electricity.

2011

Methodology for energy audits in the framework of the Energy Efficiency Directive

François Maréchal, Elfie Marie Méchaussie, Greta Martha Van Eetvelde

The Energy Efficiency Directive 2012/27/EU (EED) was released in October 2012 and transposed in June 2014 by Member States. The Directive requires large companies to carry out an energy audit before December 2015, which has to be repeated every 4 years. A possibility for companies to be exempted from regular energy audits is to be or become certified by an approved energy management system (EnMS), most likely the international standard ISO 50001. In both cases it means that companies have to set plans and define actions to comply with European and national requirements that aim at improving their energy efficiency. Considering the differences across European countries regarding the awareness and involvement of the industrial sector in terms of energy management, a large number of companies still lack systematic and comprehensive systems to understand, monitor and improve their energy consumption in a cost-effective and sustainable way. This paper presents a methodology to carry out indicative energy audits in compliance with the European standard EN 16247-1 and including the ISO 50001 requirements of the energy planning phase (e.g. energy review, energy baseline and energy performance indicators). The proposed methodology follows a top down approach, starting from the energy bill and identifying major energy sources. It covers the evaluation of the actual system’s energy efficiency, identifies energy savings opportunities and presents an innovative approach for energy consumption monitoring via surrogate models of processes. It makes use of state-of-the-art techniques such as data reconciliation, heat integration via total site pinch analysis and statistical tools. Since natural gas and electricity usually take up the largest share of the total energy consumption in industry, the focus is put on these two energy carriers. One of the interesting aspects of the methodology concerns the data gathering and processing phases. Here the required data are targeted and classified in a systematic way in order to characterise the energy consumers and identify the areas of significant energy use presenting a potential for energy efficiency improvement. Once the energy review step is carried out, strategies for energy consumption monitoring should be developed. The methodology proposes an innovative approach to generate specific energy consumption models of industrial processes (surrogate models) that could be used to monitor units, online or offline, and detect deviations from expected behaviour.

2015