By Insha Khan
Introduction
Atmospheric CO2 levels are projected to reach 550 ppm, nearly double pre-industrial levels, by 2050 if emissions remain unchecked. CO2 emissions include those from industry. Reducing such emissions would mitigate global temperature rises due to the greenhouse effect. CO2 releases into the atmosphere can be minimised by separating CO2 from post-combustion gases. This can happen through membrane-based gas separation. The effectiveness of membrane-based gas separation depends on the specific membrane. Asymmetric flat sheet hollow fiber MMMs (mixed matrix membranes) made of 6FDA-Durene and Mg-MOF-74 filler utilising OMS mechanism are the best separation method for CO2 from flue gas in industry while maximising sustainability.
Literature Review
Membrane based gas separation has emerged as a promising alternative to other forms of traditional separation such as cryogenic distillation or amine scrubbing. However, there is still debate on exactly which form of membrane is best suited for industrial grade gas separation. Initial research and studies focused on polymeric membranes being used to separate gases such as nitrogen and oxygen, discussing trade offs between permeability and selectivity, leading to knowledge of the Robeson upper bound, which defined performance limits for polymeric materials. In the 21st century, research has developed to include membranes made of inorganic materials such as zeolite and silica, and mixed matrix membranes, which are formed by a mixture of both polymeric and inorganic materials. Various applications of membrane separation have also been found over the decades, such as carbon capture and storage, and hydrogen purification. However questions remain about integration with other separation systems, sustainability and trade offs between performance and cost. This research aims to address carbon dioxide separation from flue gas through an industrial lens, taking into account industrial considerations into choice of separation method. It is secondary qualitative research, with no experiments conducted
Inherent advantages of membrane based gas separation
Cryogenic distillation and amine scrubbing are the industry standard for membrane based gas separation. Membrane-based gas separation is an emerging technology that holds several advantages over cryogenic distillation and amine scrubbing. Amine scrubbing uses aqueous amine solutions to absorb CO2 in an absorber column and then heating the solvent to release CO2 for further storage or use. Cryogenic distillation involves separating CO2 from other gas streams by cooling CO2 to low temperatures to liquefy or desublimate the gas for storage and further use. It is mature and has a capture efficiency of 90+. Cryogenic distillation makes CO2 convenient for further use, storage or transportation, while separating CO2 with 99.9% accuracy. These benefits are contested by lack of extra agents such as amine solvents required in membrane based gas separation and environmental sustainability which can be promoted through membrane based gas separation, which consumes less electricity and fuel. Other advantages offered by membrane based gas separation include simple installment and operation, adaptability of membranes for specific separation purposes, and integration of membranes with other separation methods to form hybrid separation processes to increase purity of CO2 post separation, making membrane based gas separation the best choice in industrial CO2 separation.
MMMs for CO2 separation
Before MMMs, the two main categories of membrane were polymer and inorganic membranes. Polymer membranes possess an open structure which facilitate diffusion of gases, resulting in high permeability but low selectivity while inorganic membranes offer excellent selectivity but moderate permeability. Both membranes have weaknesses. Inorganic membranes are brittle and polymer membranes are easily mechanically deformed. This can be debilitating in industrial contexts, establishing MMMs as the best choice. Membrane-based separation operates on permeability differences, using the molecular sieving or solution diffusion mechanism. Molecular sieving involves gas separation based on molecular size. When using molecular sieving, smaller molecules are able to permeate through micropores of the membrane whereas larger molecules are held back. This is the main mechanism employed in inorganic separation membranes. The solution diffusion mechanism, applied mainly in polymer membranes, comprises three steps. Firstly, the gas is absorbed into the membrane, secondly it diffuses across the membrane and thirdly, the gases desorb at the permeate side. MMMs also use the solution diffusion mechanism, combining a polymer matrix with inorganic filler to increase selectivity and rate of diffusion. They can also employ other diffusion principles depending on separation requirements. Research indicates that MMMs could surpass the Robeson upper bound, which defines the trade-off between permeability and selectivity, indicating superiority of MMMs in an industrial context.
Utilisation of MOFs
MOFs are porous crystalline solids, composed of inorganic metal ion nodes held together in a 3D shape. Membranes utilising MOFs usually apply the solution diffusion principle which, due to the tunable and customisable nature of the pore sizes, are able to easily discriminate between component gases of flue gas on the basis of size. Certain MOFs are structurally sensitive and flexible, and may undergo changes under certain temperatures and pressures, allowing for gated CO2 adsorption. Amines can be added to MOFs to chemically bond to CO2 (electron rich nitrogen in amines experiences great attraction to CO2), through introduction of Lewis basic sites. OMS (open metal sites) incorporation is another strategy that can separate CO2 from flue gas. The creation of MOFs from uncoordinated metal ion nodes leaves vacancies known as OMS in the membrane, which are heavily attracted to CO2 molecules. OMS performs better practically than the other three potential separation mechanisms. This is due to its high CO2 selectivity because of electrostatic bonding and easier regeneration (compared to Lewis basic site MOFS). It’s easier to scale than molecular sieving which requires customised and ultra precise pore sizes and gated absorption.
Other configuration aspects
The membranes should be flat sheet due to the simplicity and convenience of design, comprising sheets of porous membrane material layered on top of each other. Flat sheet membranes can be symmetric or asymmetric, with mechanical instability of symmetric designs making them unsuitable for industry. In contrast, asymmetrical hollow fiber MMMs have a high surface area to volume ratio, which can withstand high gas pressures. Polymer-filler compatibility refers to how well the MOF filler binds to polymer. Good compatibility prevents defects from forming in the membrane and promotes mechanical strength, making compatibility an important consideration. The MOF filler chosen is Mg-MOF-74. Mg2+ is a smaller cation, which means that it has a stronger electrostatic attraction to CO2. It having an electrostatic bond to CO2 also means that it’s strong enough to initially capture CO2 but weak enough to release when required, allowing for regeneration. This is in comparison to other MOF-74 fillers which all have less CO2 selectivity. The importance of compatibility would lead to a polymer such as 6FDA-Durene to be a natural choice.
Discussion
This research builds on existing work in material science and engineering, by combining earlier studies on permeability and gas separation with considerations such as durability, sustainability and scalability. This paper provides an informed conclusion on the best membrane choice for a specific industrial application, which can be implemented when working within that context in the future, while maintaining some semblance of sustainability. Since the cost of the ideal membrane for CO2 separation from flue gas is higher than other alternatives, economic research in membrane based gas separation for methods to reduce costs in design and utilisation would achieve significant impacts.
Rebuttal
A few significant counter arguments are initial set up cost, water sensitivity of OMS and humidity sensitivity of Mg-MOF-74. MOF fillers generally have higher initial costs than other conventionally used fillers for membrane based gas separation such as zeolite and
activated carbon. These initial costs are offset by mechanical strength and regeneration ability possessed by Mg-MOF-74 which leads to lower costs of replacement. Open metal sites are sensitive to water i.e presence of water can impede functionality. However, through introduction of hydrophobicity by modifying the MOF, it can physically prevent water molecules from making contact with the external or internal surface. Mg-MOF-74 is more sensitive to humidity than any other MOF-74 filler, which poses a challenge since flue gas can be humid. However this can also be nullified by introduction of hydrophobicity. Other separation technologies are considered more mature than membrane gas separation and have higher TRL ratings, however this argument overlooks how industrial applications drive
maturity increases. Mature technologies are more likely to become outdated and redundant than less mature technologies.
Conclusion
In conclusion, asymmetric flat and hollow fiber MMMs made of 6FDA-Durene and Mg-MOF-74 filler, which utilise the OMS mechanism, are the best choice for separation methods of CO2 from flue gas in an industrial context, while maximising sustainability. This is due to advantages MMMs possess in an industrial environment, incorporation of MOF fillers, electrostatic bonding created between magnesium ions and CO2 and the importance of polymer-filler compatibility in MMMs.
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