Abstract
We performed classical molecular dynamics (MD) simulations to understand the mechanism of adsorption from a gas mixture of CO2 and H2 (mole fraction of CO2 = 0.30) and diffusion along a graphite surface, with the aim to help enrich industrial off-gases in CO2, separating out H2. The temperature of the system in the simulation covered typical industrial conditions for off-gas treatment (250–550 K). The interaction energy of single molecules CO2 or H2 on graphite surface was calculated with classical force fields (FFs) and with Density Functional Theory (DFT). The results were in good agreement. The binding energy of CO2 on graphite surface is three times larger than that of H2. At lower temperatures, the selectivity of CO2 over H2 is five times larger than at higher temperatures. The position of the dividing surface was used to explain how the adsorption varies with pore size. In the temperature range studied, the self-diffusion coefficient of CO2 is always smaller than of H2. The temperature variation of the selectivities and the self-diffusion coefficient imply that the carbon molecular sieve membrane can be used for gas enrichment of CO2.
Highlights
The production of cheap membranes for CO2 gas separation purposes is of primary importance for the realization of carbon capture and sequestration technologies (He et al, 2009; He and Hägg, 2011, 2012)
INTERACTIONS BETWEEN CO2/H2 AND THE GRAPHITE SURFACE Figure 2 shows the adsorption energy profile of a single CO2 or H2 molecule on the graphite surface calculated with the force fields (FFs) and Density Functional Theory (DFT) methods for the optimum molecule-surface distance
The results show that there is a preferential adsorption of CO2 to H2 in the adsorbed layer, which depends on the temperature
Summary
The production of cheap membranes for CO2 gas separation purposes is of primary importance for the realization of carbon capture and sequestration technologies (He et al, 2009; He and Hägg, 2011, 2012). Pressure swing adsorption (PSA) is one of the most common techniques to capture CO2 from a mixture of CO2 and H2. This process requires large pressures, being different in the adsorption and desorption steps (Bernardo et al, 2009). CO2 absorbs strongly into the carbon material at high pressure. By using a molecular sieve membrane the separation can be performed as a continuous process, where the CO2 is removed both by adsorption and diffusion from the high pressure side (feed side) to the low pressure side (permeate side). To provide an energy efficient design, we will need knowledge of molecular behavior, in particular of the selectivity and of transport properties at selected process conditions. This work aims to provide such knowledge for a simplified, graphitic membrane, laying the grounds for more realistic future studies
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