Adsorption and diffusion of nitrogen, methane and carbon dioxide in aluminophosphate molecular sieve AlPO 4 -11

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Adsorption and diffusion of nitrogen, methane and carbon dioxide in aluminophosphate molecular sieve AlPO4-11 ´ gueda • M. A. Uguina J. A. Delgado • V. I. A J. L. Sotelo • Paz Ferna´ndez



Received: 8 September 2012 / Accepted: 21 December 2012 / Published online: 8 January 2013 Ó Springer Science+Business Media New York 2013

Abstract The knowledge about the adsorption and diffusion properties (specially about diffusion) of aluminophosphate molecular sieves is very scarce in the literature. These materials offer interesting properties as adsorbents as they have a polar framework and do not contain chargebalancing cations. In this work, the adsorption isotherms of nitrogen, methane and carbon dioxide over an AlPO4-11 sample synthesized in our laboratories have been measured with a volumetric method at 25, 35, 50 and 65 °C over a pressure range up to 110 kPa. The adsorption capacities of each gas are determined by the strength of interaction with the pore surface (carbon dioxide [ methane [ nitrogen). The equilibrium selectivity to carbon dioxide is quite high with respect to other adsorbents without cations due to the polarity of the aluminophosphate framework. The adsorption Henry’s law constants and diffusion time constants of nitrogen, methane and carbon dioxide in the synthesized AlPO4-11 material have been measured from pulse experiments. A pressure swing adsorption (PSA) process for recovering methane from a carbon dioxide/methane mixture (resembling biogas) has been designed using a dynamic model where the measured adsorption equilibrium and kinetic information has been incorporated. The simulation results show that the proposed process could be simpler than other PSA processes for biogas upgrading based on cation-containing molecular sieves such as 13X zeolite, as it can treat the biogas at atmospheric pressure, and it requires a lower pressure ratio, to produce high purity methane with high recovery.

´ gueda  M. A. Uguina  J. A. Delgado (&)  V. I. A J. L. Sotelo  P. Ferna´ndez Department of Chemical Engineering, Universidad Complutense de Madrid, 28040 Madrid, Spain e-mail: [email protected]

Keywords Carbon dioxide  Methane  Nitrogen  AlPO4-11  Adsorption  Diffusion  Pressure swing adsorption List of symbols b Adsorption affinity (Pa-1) b0 Pre exponential constant (Eq. 1) (Pa-1) c Adsorptive concentration in the gas phase (mol m-3) C Total gas concentration (mol m-3) cp,g Gas heat capacity at constant pressure (J mol-1 K-1) cp,s Adsorbent heat capacity (J kg-1 K-1) cv,g Gas heat capacity at constant volume (J mol-1 K-1) Dc Intracrystalline diffusivity (m2 s-1) DL Axial dispersion coefficient (m2 s-1) Dm Molecular diffusivity (m2 s-1) fpulse Function defined in Eq. (13) hext Wall to air heat transfer coefficient (W m-2 K-1) hw Gas to wall heat transfer coefficient (W m-2 K-1) Kc Dimensionless Henry’s law constant kf External mass transfer coefficient (m s-1) kmacro Combined mass transfer coefficient in the external film and the macropores (m s-1) ks LDF mass transfer coefficient (s-1) L Bed l