(649c) Molecular Simulations of Wet Flue Gas Adsorption on 13X Zeolite

Molecular Simulations of Wet Flue Gas
Adsorption on 13X Zeolite

Mark Purdue,^1,2 Zhiwei Qiao,¹
Jianwen Jiang,¹ Shamsuzzaman Farooq^1,2

¹ Department of
Chemical and Biomolecular Engineering, National University of
Singapore, Singapore 117585

² Cambridge Centre
for Advanced Research in Energy Efficiency in Singapore (CARES), CARES at
CREATE Tower. #05-05, 1 CREATE Way, Singapore, 138602

Adsorption technology
is one of the leading options currently being explored for carbon dioxide
capture and concentration (CCC) from flue gas. Among the adsorption based
processes, Vacuum Swing Adsorption (VSA) on 13X Zeolite has been widely studied
for CCC from (synthetic) dry flue gas containing 10-15% CO₂ in
balance nitrogen. However, real flue gas from any source will always contain
moisture. Competition between H₂O and CO₂ for adsorption
in 13X Zeolite has a large impact on the VSA process performance, leading to
considerations for the use of a guard bed adsorbent such as silica gel or
alternative processes to dehydrate the feed gas to a suitable extent.
Sacrificing an initial water penetration depth of 13X Zeolite in a single stage
VSA process column was recently considered using approximate equilibrium data
in VSA optimization studies. Despite the sensitivity of CO₂ capture
performance of 13X Zeolite to moisture, wet flue gas on 13X has been addressed
only in a few studies using extrapolation of limited available equilibrium data
for CO₂/H₂O mixtures. An objective techno-economic
evaluation of VSA processes with new materials in varying humidity levels
requires a validated simulation model. Process simulation model validation
requires availability of sufficient adsorbent material. A process model using a
dual-adsorbent VSA process with adsorbents Silica Gel - 13X Zeolite for CCC
from wet flue gas can be validated to establish a cost baseline for assessing
new materials and process developments. An essential pre-requisite for that is
availability of detailed equilibrium data for CO₂/N₂/H₂O
mixtures on 13X Zeolite, which was addressed in this study.

Grand Canonical Monte
Carlo molecular simulations for the adsorption of CO₂, N₂
and H₂O and associated gas mixtures on 13X Zeolite were performed. The
25Angstrom unit cell of 13X Zeolite adopted contained 83 mobile Na⁺
cations, corresponding to a Si/Al ratio of 1.31. Unlike H₂O, CO₂
and N₂ were blocked from entering smaller beta cages using 8 virtual
4Angstrom radius
blocking spheres per unit cell, which is illustrated in Figure 1 for N₂.
Intermolecular interactions between fixed charge models of adsorbate and
adsorbent consisted of 12-6 Lennard-Jones and Coulombic potentials. The isotherms
of pure components at 298, 323, and 348 K in the range 0 to 1 atm for CO₂
and N₂, and from 0 to P^sat_H2O(T) for H₂O
were initially simulated and compared to experimental data. Multicomponent
adsorption was subsequently investigated at these temperatures and over the
full range of compositions to study the influence of water vapour on CO₂
capture equilibrium. This study provides simulated isotherms for comparison
with experimental adsorption measurements by both thermogravimetric analysis
and dynamic column breakthrough approaches. A mathematical adsorption isotherm
model, which characterizes the adsorption of wet flue gas mixtures on 13X
Zeolite, is presented to facilitate accurate scale-up of VSA processes for the
capture of CO₂ from real flue gas.

Figure 1: N₂
adsorption (100kPa, 298K) on 13X Zeolite with beta cage blocking