(35a) Modeling CO2 Capture in Liquid-Amine Infused Surfaces

The release of carbon dioxide into the atmosphere from the burning of fossil fuels is considered one of the primary contributors to atmospheric CO₂ concentrations linked to climate change. Efficient and high-capacity technologies that can remove CO₂ from point sources will be a necessary tool for reducing atmospheric CO₂ levels.

In commercial practice, the usual design for CO2 capture is to use aqueous amine solutions in absorber towers configured to provide large surface areas (per unit volume) to be wet by the aqueous amine for contacting with the CO₂-containing gas streamed through the tower. Other technologies under development to provide large interfacial area for contacting the gas with amine include adsorbents in which amine molecules are grafted to surfaces, or mesoporous solids in which the interior surfaces are coated with ultrathin layers (order nanometers in thickness) of amines.

We present a technology, developed at ExxonMobil, which takes advantage of the liquid infused surfaces approach and additive manufacturing to fabricate flow-through devices with microstructures that have large interfacial surface areas per unit volume, far exceeding the values for commercial CO₂ scrubber towers. The chemistry of the surfaces of the microstructure are designed to be readily wet by pure amine liquids to form wetting infused layers, retained by capillary forces, of order 100 micrometers in thickness. We refer to this new technology as solid with infused reactive liquid (SWIRL). This large thickness and the use of pure amines in SWIRL, provides an increase in capacity for CO₂ absorption per unit volume relative to the sorbents and mesoporous materials under development.

In this presentation, we detail a transport model to describe the CO₂ capture for a flow-through geometry of a SWIRL consisting of a bundle of long parallel fibers with microstructured surfaces where the gas stream passes parallel to the axis of the fibers. The CO₂ gas stream comes into direct contact with the thin liquid amine film on the surface of these fibers and reacts to form a carbamate. The reaction in the liquid phase for this primary amine to form carbamate is taken as second order in the amine and first order in the CO₂ concentration, and the back reaction is first order in the carbamate. Diffusion of the CO₂ into the amine wetting layers of SWIRL, which become converted to carbamate, is modelled using concentrated species Stefan-Maxwell (SM) equations with diffusion coefficients formulated using the Darken and mixture self-diffusivity approximations to relate the mixture viscosity to the diffusion coefficients. A composition dependence partition coefficient describes the solubility of CO₂ in the amine/carbamate layer at the interface. Numerical solutions detail the capture of the CO₂ to form carbamate in the layer as a function of time, concentrations of CO₂ in the gas stream, reaction kinetic parameters, and solution viscosities and partition coefficients. The principal effect which governs the conversion as a function of time is the development of a highly viscous layer of carbamate at the gas/amine liquid interface. This viscous interfacial layer can act as a barrier that prevents further CO₂ diffusion into the layer and reaction with fresh amine diffusing to the surface. The simulations are compared to experiments to explain the mass transport in the system and to optimize the process in a cyclic scheme of capture and regeneration. These simulations are presented as a function of temperature.