Highly co2-selective membranes for carbon capture from flue gas: from membrane structure design and fabrication, characterization, performance evaluation, process simulation, to scale-up

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Li, Huanghe
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Electronic thesis
Chemical engineering
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Membrane-based CO2 separation technology has been considered as an up-and-coming technology due to its high energy and cost-efficiency, in pursuit of the US Department of Energy (DOE) target of 90% CO2 capture rate with >95% CO2 product purity and less than 35% increase of levelized cost of electivity for flue gas carbon capture. Facilitated transport membranes (FTM) for CO2 capture are breaking the Robeson upper bound tradeoff between CO2 permeance and CO2/N2 selectivity for polymeric membranes by their superior separation performance, especially for those containing CO2-philic groups, such as polymeric amines, multi-amines, and ionic liquids as CO2-carriers. However, state-of-the-art FTMs encounter challenges towards the flue gas carbon capture application: a) insufficient CO2/N2 selectivity to reach one-stage CO2 enrichment to 95% purity (dry-base); b) tolerance of elevated temperature range (60 – 80 ºC) and high humidity; c) long-term performance stability when treating humidified CO2 flue gas sources at elevated temperature range in practical vacuum pressure operating conditions.In this doctoral dissertation work, my focus was on the development of highly and ultra-high CO2-selective FTMs, including investigation of membrane recipes and structure, CO2 separation performance optimization under practical operating conditions, a process simulation study of two-stage membrane-based process, and the fabrication scale-up towards 1,000-cm2 bench-scale by a facile spray-coating method. First, we designed and fabricated a polystyrene sulfonate (PSS) stabilized polyethyleneimine (PEI) membrane by a facile and scalable spray-coating method. The deposited defect-free selective layer, in which the amine carriers in PEI can be stabilized electrostatically by PSS, exhibited superior CO2 separation performance with good long-term stability under practical operating conditions. The separation performance was optimized by spray-coating cycles, CNT network loading, and PSS loading. Our membrane showed CO2 permeance ranging from 820 to 1,770 GPU (Gas Permeation Unit, 1 GPU = 3.348 × 10-10 mol·s-1·m-2·Pa-1) and CO2/N2 selectivity varying from 395 to 460 under permeate side pressure (Pp) of 0.40 bara (bar absolute) in the temperature range between 80 and 90 ºC. Furthermore, the membrane was successfully scaled up to 200 cm2 with good uniformity by randomly selected testing spots. These results suggested a novel membrane structure and provided a scalable approach for fabrication of highly efficient CO2 separation membranes. Further, we designed and fabricated a novel FTM structure containing an ionic liquid (1-ethyl-3-methylimidazolium aminoacetate, [Emim][Gly]) as mobile CO2-carrier and the PEI as fixed CO2-carrier. The fixed carrier was confined within a CNT framework of 230 nm thickness via electrostatic forces adjusted by the PSS, while the mobile carrier diffuses freely within the CNT framework. After optimization of the membrane recipe and spray-coating fabrication procedure, the resulting CNT-PSS-PEI~IL membranes demonstrated an ultra-high CO2/N2 selectivity up to 1,000 with CO2 permeance up to 2,400 GPU under 70 ºC and Pp of 0.20 bara. Furthermore, one 100-cm2 flat sheet membrane sample was prepared and exhibited one-stage CO2 enrichment from 15% to 95% purity (dry-base) for the first time amongst all reported CO2 separation membranes. The membrane retained a stable performance over 50-h operation period under vacuum condition. The extraordinary CO2 separation performance illustrates the great potential of the CNT-PSS-PEI~IL membranes for flue gas carbon capture application. In addition, we proposed a two-stage membrane-based process design targeting economic carbon capture from coal-fired flue gas. This process design features the utilization of highly CO2-selective membranes for one-stage CO2 enrichment to 95% dry-base purity in the first stage and recycle of the remaining CO2 by a highly CO2-permeable membrane in the second stage, in order to achieve economic CO2 capture with 90% capture rate and >95% CO2 product purity. Through an integration-iteration membrane model and the Aspen Plus process simulation, a sensitivity study of operating pressures (feed and permeate pressures) and membrane properties (CO2 permeance and CO2/N2 selectivity) was conducted. Critical CO2/N2 selectivity of 300 - 400 was found for the highly CO2-selctive membranes to meet the demand for cost and energy efficient results. The lowest possible membrane area of 4.8 × 10^5 m2 and fractional energy of 19.3% were obtained, which is comparable to or even more attractive than reported membrane-based process designs. This work provides a new membrane process design option for highly CO2-selective membranes and gives insights on the influence of membrane performance and operation condition. Last, we proposed a continuous work of our facile and scalable lab-built spray-coating method for fabrication scale-up towards 1,000-cm2 bench-scale on our previously established ultra-high CO2-selective membranes made of CNT and ionic liquid. After fine optimization of CNT and IL loading, as well as modification of the spray-coating and spray-soaking process, the resulting 1,000-cm2 flat sheet membranes successfully demonstrated one-stage CO2 enrichment to 95% (dry-base) from simulated coal-fired flue gas feed and the reasonably decent membrane uniformity quality. These results marked a milestone of 1,000-cm2 bench-scale flat sheet ultra-high CO2-selective membrane fabrication and provided a solid base towards future practical application.
School of Engineering
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Rensselaer Polytechnic Institute, Troy, NY
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