Quite often during the operation of a system for the treatment of gases, it is necessary to expand it for treating greater streams. In some cases, future expansion is contemplated even during the initial phase of a project. In other cases, it could be a necessity not foreseen during system design phase (Miller and Stöcker 1989; Brunetti et al. 2010). Membrane system expansion is very easy, since this only requires the addition of identical modules. This is the advantage offered by the modularity of membrane units and the reduced equipment and control systems required for operating it. In comparison, considering the other reference technologies for gas separation, PSA and absorption systems can also be expanded, but it requires additional design considerations and adds cost in the initial phase of the project. The cryogenic units cannot be expanded if it is not foreseen during the design phase. Generally they can be over-dimensioned, and a capacity increase is often obtained without modification to the cold box itself through addition of a tail gas compressor. Effective diffusivity is a convenient parameter which is introduced when diffusion takes place in non-homogenous media. Consider, for example , the case where a species is diffusing through porous particles, such as reactant diffusing inside a catalytic solid. In this case, the molecules have to travel for a longer distance given that the pores of the catalytic particles are not straight, and moreover diffusion takes place over a smaller area due to the solid being wall impermeable. These effects are taken into account by defining an effective diffusivity as D eff ¼ D e t
Selective sorption of volatile aromatic compounds is a challenging issue for their total abatement. Despite the well-known affinity of palladium toward rich π systems, studies dedicated to volatile organic compound (VOC) capture with Pd(II)-based metal−organic frameworks (MOFs) are still very scarce. Intending to shed more light on this complex topic, this work compares the adsorption properties of two isostructural MOFs [Cu(2-pymo) 2 ] n and [Pd(2-pymo) 2 ] n and their selectivity for the sorption of linear, cyclic, or aromatic VOCs. The combination of both experimental and computational investigations highlights an increasing aromatic affinity over saturated hydrocarbons when palladium is chosen as a metal center (n Benzene /n n-hexane = 1.8 at 0.5 p/p 0 ) in the MOF instead of copper (n Benzene /n n-hexane = 0.7 at 0.5 p/p 0 ). Furthermore, [Pd(2pymo) 2 ] n clearly exhibits preferential adsorption of benzene over toluene (n Benzene /n Toluene = 1.7 at 0.5 p/p 0 ), due to the steric hindrance effects of the latter. The present results clearly underline the attractiveness of Pd-based MOFs for the design of selective aromatic adsorbents. Moreover, they also highlight the [Pd(2-pymo) 2 ] n MOF as a relevant candidate for the selective capture of benzene, by a synergistic combination of both charge interactions and steric hindrance effects.
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