A database of permeability, diffusivity and solubility for He, H2, O2, N2, CO2 and CH4 was compiled from the literature for rubbery polymers. These data were compared with results for a similar study ...conducted for glassy polymers published in 2013. Based on this comparison, glassy polymers have higher solubility coefficients than rubbery polymers due to the excess volume (or free volume) stemming from the non-equilibrium nature of polymers below the glass transition temperature, Tg, which is well described by the dual-mode sorption model. When compared at equal permeability, rubbery polymers have higher diffusion coefficients and lower solubility coefficients. Polymers having permselectivity values at or near the upper bound are virtually all glassy polymers. This phenomenon is widely ascribed to the better diffusivity selectivity of glassy versus rubbery polymers. Comparison of the upper-bound plot of log Pi/Pj versus log Pi (where Pi represents the permeability of the more permeable gas) shows that glassy polymers dominate the upper bounds for all 15 gas pairs possible. However, when log Pi/Pj is plotted versus log Di, many of the gas pairs have overlapping data for glassy and rubbery polymers. Thus, glassy polymers dominate the upper bound due, in part, to their higher solubility coefficients.
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•Rubbery polymers typically have lower solubility coefficients than glassy polymers.•Rubbery polymers generally have higher diffusion coefficients than glassy polymers.•Diffusion selectivity advantage of glassy polymers is more modest than suggested.•Rubbery polymers have modestly low dependence on diffusion selectivity.•Glassy polymers dominate the upper bound due to higher solubility coefficients.
In this paper, heat transfer enhancement in a parabolic trough receiver using wall-detached twisted tape inserts was numerically investigated. The resulting heat transfer, fluid friction and ...thermodynamic performance were determined and presented. The flow was considered fully developed turbulent, with Reynolds numbers in range 10 260 ≤ Rep ≤ 1 353 000 depending on the fluid temperature. The twisted tape's twist ratio and width ratio vary in the range 0.50–2.00 and 0.53–0.91, respectively. The numerical investigations are based on a finite volume method, with the realisable k–ɛ model for turbulence closure. The study shows considerable increase in heat transfer performance of about 169%, reduction in absorber tube's circumferential temperature difference up to 68% and increase in thermal efficiency up to 10% over a receiver with a plain absorber tube. An entropy generation analysis shows the existence of a Reynolds number for which there is minimum entropy generation for each twist ratio and width ratio. The optimal Reynolds number increases with increasing twist ratio and reducing width ratios. The maximum reduction in the entropy generation rate was about 58%. Correlations for heat transfer and fluid friction performance for the range of parameters considered were also derived and presented.
•Study on the use twisted tape inserts in a parabolic trough receiver is presented.•Use of twisted tape inserts significantly improves receiver thermal performance.•Receiver thermal efficiency increases up to 10% with twisted tape inserts.•At larger Reynolds numbers use of inserts degrades receiver thermodynamic performance.•Entropy generation rates reduce up to 58% with the use twisted tape inserts.
Membrane gas separations require materials with high permeability and good selectivity. For glassy polymers, the gas transport properties depend strongly on the amount and distribution of free ...volume, which may be enhanced either by engineering the macromolecular backbone to frustrate packing in the solid state or by thermal conversion of a soluble precursor to a more rigid structure of appropriate topology. The first approach gives polymers of intrinsic microporosity (PIMs), while the second approach is used in thermally rearranged (TR) polymers. Recent research has sought to combine these approaches, and here a new range of thermally rearrangeable PIM-polyimides are reported, derived from dianhydrides incorporating a spiro center. Hydroxyl-functionalized polyimides were prepared using two different diamines: 2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane (bisAPAF) and 4,6-diaminoresorcinol (DAR). Thermal treatment at 450 °C under N2 for 1 h yielded polybenzoxazole (PBO) polymers, which showed increased permeability, compared to the precursor, in membrane gas permeation experiments. A polymer based on DAR (PIM-PBO-3) exhibited a CO2/N2 selectivity of 30 as prepared, higher than the values of 21–23 obtained for polymers derived from bisAPAF with the same dianhydride (PIM-PBO-1).
Metal-organic frameworks (MOFs) for hydrogen storage have continued to receive intense interest over the past decade. MOFs are a class of organic-inorganic hybrid crystalline materials consisting of ...metallic moieties that are linked by strong coordination bonds to organic ligands. They exhibit a great structural diversity and possess low weight, exceptionally high surface areas, large free volumes, and tunable pore sizes and functionalities, making them extremely attractive for a variety of applications such as hydrogen storage. For these reasons MOFs have been extensively studied. In this paper, a review of recent developments on hydrogen storage in MOFs is presented, with a focus on the effects of various factors including open metal sites, ‘guest’ metal ions, ligand functionalization, surface area, pore volume, pore size, and Pt or Pd metal nanoparticles, on hydrogen storage. In addition, the review examines the emerging research on MOF hybrid hydrogen storage systems, primarily in the context of employing MOFs for nanoconfinement of high temperature hydrogen storage materials. The review focuses on experimental studies.
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