(149h) Intensification of CO2 Utilization in Membrane Reactors

Carbon dioxide emissions from the combustion of fossil fuels and other chemical processes (such as natural gas reforming for hydrogen production) are the key contributor to climate change and the global warming phenomenon. The main viable approach to mitigate these harmful impacts is carbon utilization. This approach leads to a sustainable material cycle (assuming the energy for the process originates from renewable sources). Converting captured CO₂ into useful products is the most rational way to realize a carbon neutral industry, by which CO₂ acts as a cheap (actually it has a negative cost due to carbon taxes), nontoxic and abundant C1 source.

The main acceptable route to produce renewable chemical and fuels from CO₂ includes three main reaction steps. Green power generated on site (e.g. in a hydroelectric power station, or from photovoltaic cells) generates hydrogen and oxygen from water by electrolysis. In the next step the hydrogen reacts with CO2 according to the reverse water gas shift reaction, Eq. (1), to eventually form long-chain hydrocarbon compounds via to the exothermic Fischer-Tropsch reaction in the third reaction step.

CO₂ + H₂ ⇌ CO + H₂O, ∆H₀ = 41.2 kJ/mol (1)

In-situ separation of steam, is a promising process intensification approach for many types of reactions in which water is formed as a byproduct [1,2]. Hydrophilic inorganic materials have been proposed for selective removal of the polar steam molecule at elevated temperatures (200-550 â—¦C) from mixtures containing other gases (e.g. H₂ and CO).

In this contribution we study the potential intensification of reverse water gas shift reactors by integrating water selective membranes in various structures and configurations. We demonstrate rigorous mathematical models which enable model-based design of such membrane reactors and elucidate the potential of novel materials and process concepts. The models incorporate physical parameters based on reported empirical data. We quantitatively evaluate the concept of sweeping the permeate side of the membranes with the dried product stream to increase driving force for water permeation and at the same time reduce it for other species. The improved performance of such structured membrane reactors is outlined and discussed in detail. Significant improvement of CO₂ conversion is achieved even for modest membrane selectivity (water vs H₂ and CO) values.