Fixed bed adsorption of benzothiophene over zeolites with faujasite structure
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Fixed bed adsorption of benzothiophene over zeolites with faujasite structure J.L. Sotelo · M.A. Uguina · V.I. Águeda
Received: 27 April 2007 / Revised: 19 July 2007 / Accepted: 23 July 2007 / Published online: 20 September 2007 © Springer Science+Business Media, LLC 2007
Abstract This work addresses the adsorption of benzothiophene (BT), as a model heterocyclic and aromatic sulphur compound present in road fuels, over agglomerated zeolites with faujasite structure. Several adsorbents based on zeolites with FAU structure have been prepared with different Si/Al molar ratios and exchange cations and then agglomerated. The influence of the zeolite basicity has been studied, both in equilibrium and dynamic liquid phase adsorption experiments. Basicity of the adsorbent increased as the Si/Al molar ratio and the electronegativity of the exchange cation decreased. In equilibrium experiments, the affinity towards the adsorbent increased as the Si/Al molar ratio decreased, showing the highest affinity for exchanged low silica X zeolites with medium basicity (A-KLSX-02). Dynamic experiments showed that the less zeolite basicity, the higher fractional bed utilization and adsorption capacity at breakthrough time. Besides, zeolites with high basicity did not reach the equilibrium capacity due to the low diffusivity of BT into the micropores. Thermogravimetric analyses of the spent adsorbents showed a stronger BT adsorption onto the more basic zeolites. As main conclusion, adsorbents with medium basicity could present the best performance in fuel desulphurization due to their high affinity with sulphur compounds, although diffusion problems should be taken into account. Keywords Benzothiophene · LSX zeolites · Basicity · Agglomeration · Desulphurization · Adsorption · Fixed bed
J.L. Sotelo () · M.A. Uguina · V.I. Águeda Department of Chemical Engineering, Universidad Complutense de Madrid, 28040 Madrid, Spain e-mail: [email protected]
Abbreviations BT Benzothiophene C Sulphur concentration, ppmw FAU Faujasite FBU Fractional bed utilization; defined as q br /q sat K Equilibrium constant, ppm−1 ; parameter defined in (2) q Adsorption capacity, mg g−1 q D.eq Adsorption capacity from the isotherm at a given concentration, mg g−1 qmax Maximum adsorption capacity, mg g−1 ; parameter defined in (2) S Sanderson electronegativity of an element; parameter defined in (1) Sanderson intermediate electronegativity; Sint parameter defined in (1) t Time, min T Temperature, K Subscripts and superscripts 0 Initial br Breakthrough time eq Equilibrium p, q, r Stoichiometric coefficient of the atom in a unit cell of adsorbent P, Q, R Elements present in zeolite composition sat Saturation time
1 Introduction At present, European legislation limits sulphur content in road fuels to 50 ppmw. This will be reduced to 10 ppmw by 2009 (European Directive 2003/17/CE). However, sulphur concentrations below 1 ppmw have to be achieved for the
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application of transportation fuels in fuel cells technologies because s
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