(521a) Enhanced Selective Adsorption Desulfurization on CO2 and Steam Treated Activated Carbons: Equilibria and Kinetics

Selective adsorption removal of sulfur (SARS) is one of the most promising alternative strategies to ultra-deep hydrodesulfurization (HDS) for lowering the sulfur content of liquid hydrocarbon fuels and meet stringent environmental regulations. This process employs solid adsorbents and, unlike HDS, it operates at less severe temperatures and pressures, without the costly hydrogen consumption.[1,2] Activated carbons (ACs) show great potential for SARS but require improvements in uptake and selectivity.

The current work examines comprehensively the influence of modifying a commercial-grade activated carbon (AC) by CO₂ and steam treatment for the desulfurization of dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT), which are sterically hindered sulfur compounds that are particularly difficult to remove from hydrotreated gas oils by conventional HDS methods. An untreated AC and a charcoal Norit carbon (CN) are used for comparative purposes. The materials are characterised using physicochemical techniques such as N₂ physisorption, HRTEM, SEM, XRD, ATR-FTIR, ICP and TGA analyses. The equilibria and kinetics of the materials are examined using both batch and fixed-bed flow measurements, respectively, at low adsorption temperatures (303 to 353 K) and ambient pressure. Full details of the experimental techniques used are given elsewhere.[3]

The results show that the steam and CO₂ treated ACs exhibited significantly higher sulfur uptakes than the untreated AC and CN samples due to an increased surface area and microporosity. The volume of micropores governs capacities mainly by physical p-p interactions. Surface chemistry effects were found to be relatively small. The steam treated AC appears to be especially effective to remove sulphur showing a remarkable uptake (~24 mg S•gads^-1), due to its smaller molecular size which favours its diffusion and thus leads to a better developed microporosity. The modified ACs showed similar capacities to both DBT and the sterically hindered 4,6-DMDBT molecules. Compared to other adsorbents, the modified ACs exhibited relatively high adsorption capacities.[1,2,3] The adsorption sites of all the samples assessed are characterized as heterogeneous due to the better fit to the Freundlich model. The kinetic breakthrough profiles are adequately described by the linear driving force (LDF) model. The findings of this work provide very useful information for the design of commercial adsorption units. Research efforts are being devoted to detailed selectivity and stability analyses and to the use of more rigorous adsorption model approaches.

Acknowledgements

This work is supported by the ELEGANCY, EU H2020 project and the Imperial College London Research Fellowship Grant.

References

[1] D. Iruretagoyena, R. Montesano, Selective sulphur removal from liquid fuels using nanostructured adsorbents, in: T.A. Saleh (Ed.), Nanotechnology in oil and gas industries: Principles and applications, Springer International Publishing, Cham, 133-150, 2018.

[2] R. Menzel, D. Iruretagoyena, Y. Wang, S. M. Bawaked, M. Mokhtar, S. A. Al-Thabaiti, S. N. Basahel, M. S. P. Shaffer, Graphene oxide/mixed metal oxide hybrid materials for enhanced adsorption desulfurization of liquid hydrocarbon fuels, Fuel, 531-536 (181), 2016.

[3] D. Iruretagoyena, K. Bikane, N. Sunny, H. Lu, S. G. Kazarian, D. Chadwick, R. Pini, N. Shah, Enhanced selective adsorption desulfurization on CO₂ and steam treated activated carbons: Equilibria and kinetics, Chem. Eng. Journal, 122356 (379), 2020.