Analysis of CO2 hydrate formation from flue gas mixtures in a bubble column reactor

Abstract

Gigascale carbon capture and sequestration (CCS) is increasingly seen as essential to meeting the targets of the Paris Agreement. As an alternative to conventional CCS approaches, carbon dioxide (CO2) hydrates have received attention as materials which can enable new approaches to carbon capture as well as carbon sequestration. CO2 hydrates (ice-like materials of CO2 and water) form at medium pressures (<400 psi) and temperatures of >0 °C from a water-CO2 mixture. Bubble column reactors (BCR) have been studied as a preferred way of forming CO2 hydrates. This study uses an inhouse, recently-developed modeling framework to predict performance of a BCR for CO2 hydrate formation from flue gas (CO2/N2), and pure CO2 streams. We highlight and analyze specific aspects of hydrate formation that are important for CO2 sequestration, and for CO2 separation/capture. In particular, two performance parameters are analyzed: i) gas consumption rate for hydrate formation (normalized with reactor volume), and ii) fraction of CO2 that converts to CO2 hydrates in a single pass (conversion factor). The first metric quantifies the overall productivity of a BCR by obtaining the net CO2 that can be sequestered or separated from the flue gas stream. The second metric relates to the efficiency of the system by quantifying the need for recirculation and the quality of the exit stream after a single pass. Extensive parametric analysis is conducted to study the influence of pressure, temperature, CO2 mole fraction at inlet, gas flow rate and reactor geometry on hydrate formation. Across the range of simulations conducted in this study, the highest gas consumption rate per unit reactor volume was 28.9 ton/yr/m3 and the highest conversion factor was 67.8 %. Both parameters increase with increasing pressure, decreasing temperature and increasing inlet mole fraction of CO2. Increasing gas flow rate increases the gas consumption rate (i.e., hydrate formation rate) but reduces the conversion factor. This suggests that the overall productivity of BCRs increases with gas flow rate at the expense of its efficiency. Reduced efficiency increases recirculation-related costs and high flow rate increases compression and cooling costs. For flue gas, increasing the reactor volume by increasing the height or diameter increases conversion factor but significantly reduces the gas consumption rate per unit reactor volume. For pure CO2, increasing the reactor height increases the conversion factor without changing the volumetric gas consumption rate. Decreasing the diameter increases volumetric gas consumption rate without changing the conversion factor. These findings suggest that compact reactors are more suitable for CO2 hydrate slurry production (on a volumetric basis), while larger reactors are suitable for CO2 separation/capture applications. Overall, this study provides a basis for the design and operation of BCRs for CO2 hydrates-based CCS applications.