Adsorption and Diffusion of N₂ and CO₂ and Their Mixture on Silica Gel

The rising concentration of CO₂, emitted into the atmosphere from power plant flue gas, is a major contributor to global warming. Silica gel is an important adsorbent to dry wet flue gas prior to sending the dried gas (CO₂/N₂ mixture) for carbon capture. In the present work, a comprehensive experimental and simulation study is undertaken to establish the adsorption and diffusion of N₂ and CO₂ and their mixture on silica gel at pressures and temperatures relevant to vacuum swing adsorption (VSA) processes. The adsorption equilibrium of pure N₂ and CO₂ is captured well by the single component Langmuir isotherm model. Carefully designed controlled experiments are conducted to show that the transport mechanism for the adsorption of pure N₂ in silica gel pores is governed by Knudsen flow, while for CO₂, it is a combination of Knudsen and surface flow. Binary mixture experiments are performed to confirm the mixture equilibrium and kinetic models necessary to simulate the dry product end of a column in a VSA process for drying wet flue gas. For binary mixture equilibrium of these gases, there is no effect of competition from the other gas present in the mixture, implying that they exhibit noncompetitive adsorption on silica gel. Transport of CO₂/N₂ mixture in silica gel pores is well captured by the mechanism established from the single component study. ©