A numerical study is carried out on double-diffusive, natural convection within a vertical annular, porous cylinder. The flow is driven by buoyancy force due to externally applied constant temperature difference on the vertical cylinder while the horizontal surfaces are impermeable and adiabatic. The effects of cross phenomena "Soret effect" were considered in the analysis. It is demonstrated that the cylindrical annulus permits a higher thermal gradient. It is established that such system has an optimal separation effect for a given thermal Rayleigh number, Ra-T. To overcome such limitation, two sub-domains (buffer) allowing filtration separation was proposed and investigated. The flow field, temperature and concentration distributions are obtained in terms of the governing parameters. The effect of the sub-domains scales, the Darcy number, Da, and the curvature, R, on flow, temperature and species separation ability is found to be significant. (C) 2008 Elsevier Ltd. All rights reserved.