2-dimensional membrane separator modelling: Mass transfer by convection and diffusion

AbstractHydrogen selective membranes may present a technologically and economically efficient method for the separation of H2 from CO2 in pre-combustion decarbonisation of power production from fossil fuels. Accurate scale-up and performance prediction of membranes strongly depends on adequate representation of the prevailing resistances to mass transfer, especially for present-day high flux membranes. In a series of experiments, H2/N2 separation is measured as a function of feed flow and retentate pressure for a supported palladium membrane enclosed by annular channels for feed/retentante and sweep/permeate flow. Comparison of model predictions with measured data reveals that mass transfer resistances in the gas phase are significantly reduced by a radial velocity component in cases of high transmembrane flux, which can only be adequately described by a 2D model. For accurate interpretation of experiments, scale-up, and design of modules with high flux membranes, 2D modelling is required

Structure–Activity Correlations for TON, FER, and MOR in the Hydroisomerization of n-Butane

n-Butane hydroconversion was studied over (Pt-loaded) molecular sieves with TON, FER, and MOR morphology. The conversion occurs via a complex interplay of mono- and bimolecular bifunctional acid mechanism and monofunctional platinum-catalyzed hydrogenolysis. Hydroisomerization occurs bimolecularly at low temperatures. This is strongly indicated by the reaction order in n-butane of 2 for isobutane formation and the presence of 2,2,4-trimethylpentane among the products. Intracrystalline diffusion limitations of the reaction rates seem to be important for TON. Due to diffusion-controlled reaction rates for TON, the presence of Pt in TON was detrimental for the isomerization selectivity. As the ratio of utilized acid sites to accessible Pt becomes low (approximately 1:75), diffusion of the feed molecules to the acid sites is too slow to prevent Pt hydrogenolysis of n-butane. Reactions on H-FER occur predominantly on the outer surface and the pore mouth of the molecular sieve, presumably owing to rapid pore filling following a transient period of single-file diffusion. Due to high intrinsic activity toward (hydro)cracking this does not lead to high selectivity toward isobutane. Addition of Pt (bifunctionality) was in this case beneficial. Reaction at the external surface is not diffusion limited, allowing bifunctional nC4 isomerization to occur. Although PtFER was found to approach selectivity levels as found for PtMOR, the latter has a significant advantage as the larger concentration of accessible acid sites leads to much higher activity

Mechanistic routes of low temperature alkane activation over zeolites

Two reaction pathways for the hydrogen/deuterium exchange of iso- and n butane over zeolites MFI and BEA are proposed both having as key step the alkane dehydrogenation. The difference in the apparent activation energies over the two zeolites suggests marked differences in the nature of this step. For H-MFI the absolute rate of H/D exchange increased in parallel with the concentration of Brønsted acid sites. The H/D exchange is suggested to occur via the protonation of alkene intermediately formed by protolytic dehydrogenation. The Lewis acid sites accelerate by (a non-catalytic) hydride abstraction leading to sorbed hydrogen (atoms) and alkoxy species on the surface. Even traces of olefins accelerate the H/D exchange dramatically emphasizing their important role for the H/D exchange of alkanes

Reactive Water Sorbents for the Sorption‑Enhanced Reverse Water–Gas Shift

Sorption-enhanced reverse water–gas shift (SE-RWGS, here designated as ‘COMAX’) was studied with bifunctional reactive sorbents. First proof-of-concept is presented of the successful design of a multifunctional reactive sorbent, which combines CO2 activation and water adsorption functionalities in an integrated reactive sorbent, i.e. the active phase is loaded on the carrier that provides surface area for dispersion of the active (Pt, Cu) phase as well as H2O sorption capacity. Near complete selectivity to CO was achieved from atmospheric pressure up to at least 29 bar, i.e. the highest pressure studied in the experimental campaign. This selectivity was obtained with stoichiometric and excess quantities of hydrogen in the (RWGS) COMAX feed, the latter in view of the potential use of syngas mixtures. The newly developed bifunctional material bears important additional advantage for scaling up of the COMAX process, because it avoids the mixing of catalyst and adsorbent materials that differ in properties such as hardness. Evidently, the key parameter for optimizing the COMAX process is the working adsorption capacity of the system and (multi-column) cycle design. Improving the capacity can be done by optimizing the reactive adsorption conditions and by optimizing the regeneration method

The Work and Preliminary Results of the C4u Project on Advanced Carbon Capture for Steel Industries Integrated in Ccus Clusters

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