Session: 14-02: Carbon Capture & Cleaner Fossil Fuel Technologies II

Paper Number: 169911

169911 - Effect of Capillary and Viscous Forces on Carbon Dioxide Desorption in Porous Adsorbent Layers 

Abstract: 

Temperature swing adsorption-based (TSA) carbon dioxide (CO₂) separation processes, which involve an alternating flow of gases and liquids in adsorbent-coated microchannels, have been shown to yield superior and environmentally friendly performance than the existing processes. This technique begins by sending the impure gas mixture through hydrophobic adsorbent-coated microchannels, which results in the adsorption of CO₂. Subsequently, hot water heat transfer fluid (HTF) flows through the same channels, displaces the purified gas stream, heats the adsorbent, desorbs the CO₂, and removes that desorbed CO₂ from the channel. This stage is followed by cooling and purging of liquid in preparation for the next cycle. In this novel TSA cycle, the second or regeneration stage involves a complex and poorly studied set of transport phenomena involving multiphase interaction between hot liquid water HTF, solid adsorbent layer, adsorbed CO₂, dissolved CO₂, and gaseous CO₂. The successful development and deployment of this shared-channel adsorption-driven separation (SCAD) process hinges on a complete understanding of these phenomena. For studying this process and its feasibility, the fundamental three-phase phenomena in the adsorbent layer must be conceptualized and simulated. During this stage, the sensible heat added to the layer from the HTF water is consumed for the desorption of CO₂, generating competing heat transfer effects. As CO₂ is desorbed, it encounters the hot liquid water outside the adsorbent nanopores, and some of it is absorbed in water. This creates competing mass transfer effects of desorption from the adsorbent and absorption into water. Capillary effects and wetting phenomena in the macroscopic voids of the adsorbent layer would complicate this fluid transport, which also has not been understood in the context of desorption. For instance, capillary action is expected to drive water inside the adsorbent layer while the competing viscous forces resist any movement of these working fluids.   

The computational model in this work studies the interplay between viscous forces on the liquid HTF and desorbing CO₂, and capillary forces on the liquid HTF to track the interface location within the adsorbent layer. The goal is to accurately predict the timescales of desorption, absorption of CO₂, and wetting during the desorption stage and track the movement of the CO₂-water interface as a function of these phenomena. gPROMS ProcessBuilder is used for solving complex differential equations as well as an expansive bank of thermophysical property data. Species balance and energy balance, coupled with the momentum balance and adsorption isotherm equations, are implemented to monitor the local and temporal pressure and velocity variations within the layer. The computational models reveal a straw-like movement of liquid water through the porous adsorbent layer. To track the gas-liquid interface, the capillary action and viscous forces are considered within the momentum balance using Darcy’s law for porous media and capillary pressure. Early results show water entering the porous layer guided by capillary forces and then promptly pushed out of the pore within 6 microseconds from the increase in gaseous pressure from desorption. This insignificant delay is attributed to the heat transfer from the channel to the adsorbent layer. From these results, it can be intuited that within the porous material the effect of wetting within the adsorbent layer is minimal in the early phase of desorption. When the substantial mass of the CO₂ is rejected to the channel, then the interface begins to move inside the adsorbent layer, which is aided by the capillary effect and absorption of CO₂ into water to achieve mechanical equilibrium. This work is intended to map these time scales for all integral transport phenomena so that a feasible CO₂ desorption strategy could be devised for experimental implementation and eventual deployment of the SCAD system.

Presenting Author: Caroline Mcclung Florida Institute of Technology

Presenting Author Biography: Caroline graduated with a bachelor of arts in Mathematics from Bryn Mawr College in Bryn Mawr, Pennsylvania. After completing her bachelor's, she worked as a high school math teacher until deciding to pursue engineering. She is currently an MS student in Mechanical Engineering, graduating in the summer of 2025, and is currently studying thermal and fluid sciences. Her research focuses on computational modeling of two-phase flows in microchannels.