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In this study, DDR membranes with a layer thickness of approximately 700 nm were studied for separation feeds comprising mixtures of CO 2 and CH 4. The membranes displayed the highest CO 2 over CH 4 permselectivity and CO 2 permeability reported in literature. This was ascribed to a defect-free and ultra-thin zeolite film as well as an open and highly permeable support. For equimolar mixtures, the highest CO 2 over CH 4 permselectivity of 727 was observed when the pressure at the feed side was 5 bar(a) and the permeate pressure was 1 bar(a) at 25 °C. At these conditions, the CO 2 permeability was very high at 45 × 10−7 mol/(m2 s Pa). Separation experiments for 80/20 and 20/80 mixtures were also performed, and in these cases, CO 2 over CH 4 permselectivities of 1011 and 622 were observed, respectively. For all feeds, the membrane permselectivity decreased slightly at higher temperature and in all cases, higher permselectivity was observed when vacuum was applied at the permeate side. One-stage membrane processes for upgrading biogas to biomethane at three different operating pressures were designed based on the experimental data. In all cases, a quite low membrane area, methane slip and power need were observed. [Display omitted] • DDR membranes with ultrahigh CO 2 permeance for CO 2 /CH 4 separation. • One-stage DDR membrane processes for biogas upgrading. • Low membrane area, CH 4 slip, and power use for these processes. [ABSTRACT FROM AUTHOR]

The NET power cycle (NPC) offers an alternative way to inherently capture combustion derived CO2 with low efficiency penalty. This manuscript improves the NPC by recompressing a portion of the recycled CO2 stream to the exit of low temperate regenerator, forming a recompression NET power cycle (RNPC). The RNPC further enhances cycle recuperation and no longer needs to input extra heat to cover the heat gap between low pressure and high pressure CO2 streams in the regenerator. The model of internal convection cooling for the turbine in sCO2 cycle is also accomplished according to the semiempirical formula, aiming to investigate the effect of coolant flow. This manuscript respectively conducts the system multiparameter optimization for uncertain boundary parameters and the sensitivity analysis for definite boundary parameters. The simulation results show that RNPC can output 27.2 MWe more NEP than NPC without extra heat input, leading to 3.54% of the increase in net electric efficiency. RNPC's efficiency increases monotonically with not only turbine inlet temperature but inlet pressure, which indicates that RNPC has the potentiality to further improve its efficiency at higher turbine inlet temperature and inlet pressure. However, turbine outlet pressure has a nonmonotonic effect on the cycle performance, and the optimal turbine outlet pressure corresponding to the maximum cycle efficiency is about 38–42 bar. The multiparameter optimization indicates that the global optimal cycle efficiency is approximately 53.55% within the ranges of investigated parameters. The RNPC's efficiency is considerably influenced by the minimum cycle temperature (0.234%/°C), compressor efficiency (0.08%/%), turbine efficiency (0.355%/%), and regenerator temperature differences (0.083%/°C for HTR and 0.080%/°C for LTR). [ABSTRACT FROM AUTHOR]

We developed a series of molecule-scale hybrid amine-silica membranes synthesized from organoalkoxysilane precursors of 3-(triethoxysilyl)propan-1-amine (PA-Si) and 3-(triethoxysilyl)-N-methylpropan-1-amine (SA-Si), and made these functional using either unhindered amines or a 3-(triethoxysilyl)-N,N-dimethylpropan-1-amine (TA-Si) that is sterically hindered. CO 2 adsorption-desorption measurements of amine-silica powdered xerogels were conducted to observe the effect of amine type on CO 2 adsorption and desorption/diffusion properties. The results revealed that TA-Si xerogel powders demonstrated faster kinetics for both adsorption and desorption processes compared with those of the PA-Si and SA-Si samples due to the steric hindrance effect of the amine, which reduced the CO 2 binding energy and thereby boosted both the forward and reverse reaction rates of CO 2 -amine. In single-gas permeation performances, all the membranes exhibited excellent molecular sieving at higher temperatures and all the gases considered, except CO 2 , tended to permeate the membranes via activated diffusion. The effect of amine type on CO 2 separation performance was compared using CO 2 permeance, CO 2 /N 2 selectivity, activation energy for permeation ( E p ) of CO 2 [ E p (CO 2 ) ], and differences in E p between CO 2 and N 2 [ E p (CO 2 )-E p (N 2 ) ]. The TA-Si membrane demonstrated superior CO 2 separation performance, and achieved the highest values for both CO 2 permeance and selectivity. A relatively smaller difference in CO 2 separation performance was observed between PA-Si and SA-Si membranes despite some differences in basicity. This suggests that, rather than the basicity, it was the steric hindrance effect that played the greatest role in CO 2 transport performance across amine-silica membranes. [ABSTRACT FROM AUTHOR]

In the present work, single channel tubular zeolite CHA membranes with a length of 50 cm and a membrane area of 100 cm2 were evaluated by SEM and permeation experiments with single components and industrially relevant humid mixtures. The membranes comprised well-intergrown and smooth CHA films with a thickness of <500 nm supported on the inside of a highly porous tube. For single component permeation, a very low SF 6 permeance of 4.5 × 10−10 mol/(m2‧s‧Pa) was observed, which indicated nearly defect free membranes. On the contrary, the membranes displayed a very high CO 2 permeance of 128 × 10−7 mol/(m2‧s‧Pa), which illustrated the very high permeability of the CHA pores. Finally, the membranes displayed excellent selectivity for separation of industrially relevant CO 2 /CH 4 /H 2 O mixtures, which was attributed to selective interaction between the CO 2 molecules and the polar water molecules in the pores. The highest observed CO 2 /CH 4 separation selectivity was as high as 198 in combination with a CO 2 permeance of 14 × 10−7 mol/(m2‧s‧Pa) at a feed pressure of 600 kPa (including 2.2 kPa water) and 293K. The corresponding CO 2 flux was 0.39 mol/(m2‧s) and the corresponding CO 2 /CH 4 separation factor was 162. The observed membrane performance was reduced by concentration polarisation due to the limited feed flow in the experimental setup. The corresponding selectivity and CO 2 permeance corrected for concentration polarisation were as high as 236 and 16 × 10−7 mol/(m2‧s‧Pa), and the corrected CO 2 flux was 0.44 mol/(m2‧s) and corrected separation factor was 198. An estimate showed that even at a low feed pressure of 500 kPa, it would be sufficient with as few as 64 membranes for processing of a feed of 100 Nm3/h raw biogas, i.e. the capacity of a typical biogas plant at a large farm, to biomethane with high purity. These results illustrated that the membranes are promising candidates for industrial separation of CO 2 from e.g. natural gas and biogas. [Display omitted] • The first report described industrially relevant CHA membranes. • The membrane was a single channel tube with a membrane area of 100 cm2. • CO 2 single gas permeances was as high as 128 × 10−7 mol/(m2‧s‧Pa). • The highest observed CO 2 /CH 4 separation factor was 169. • Effect of concentration polarisation on separation was discussed. [ABSTRACT FROM AUTHOR]

Abstract: A magnesium-based metal organic framework (MOF), also known as Mg-MOF-74, was successfully synthesized, characterized, and evaluated for adsorption equilibria and kinetics of CO2 and CH4. The Mg-MOF-74 crystals were characterized with scanning electron microscopy for crystal structure, powder X-ray diffraction for phase structure, and nitrogen adsorption for pore textural properties. Adsorption equilibrium and kinetics of CO2 and CH4 on the Mg-MOF-74 adsorbent were measured in a volumetric adsorption unit at 278, 298, and 318K and pressures up to 1bar. It was found that the Mg-MOF-74 adsorbent prepared in this work has a median pore width of 10.2Å, a BET specific surface area of 1174m2/g, CO2 and CH4 adsorption capacities of 8.61mmolg−1 (37.8wt.%) and 1.05mmolg−1 (1.7wt.%), respectively, at 298K and 1bar. Both CO2 and CH4 adsorption capacities are significantly higher than those of zeolite 13X under similar conditions. The pressure-dependent equilibrium selectivity of CO2 over CH4 (q CO2/q CH4) in the Mg-MOF-74 adsorbent showed a trend similar to that of zeolite 13X and the intrinsic selectivity of Mg-MOF-74 at zero adsorption loading is 283 at 298K. The initial heats of adsorption of CO2 and CH4 on the Mg-MOF-74 adsorbent were found to be 73.0 and 18.5kJmol−1, respectively. The adsorption kinetic measurements suggest that the diffusivities of CO2 and CH4 on Mg-MOF-74 were comparable to those on zeolite 13X. CH4 showed relatively faster adsorption kinetics than CO2 in both adsorbents. The diffusion time constants of CO2 and CH4 in the Mg-MOF-74 adsorbent at 298K were estimated to be 8.11×10−3 and 4.05×10−2 s−1, respectively, showing a modest kinetic selectivity of about 5 for the separation CH4 from CO2. [Copyright &y& Elsevier]

The abatement of defects in zeolite membranes is essential for achieving high selectivity. In the present work, a simple and effective method for blocking defects in ultra-thin (ca. 0.5 μm) MFI zeolite membranes has been developed. The method is based on deposition of an ultra-thin (~15 nm) layer of amorphous silica on the top surface of the membrane. Permporometry data indicated that the amount of defects, especially defects larger than 4 nm, in the membranes was significantly reduced after the modification. In mixture separation experiments, the CO 2 /H 2 separation factor increased dramatically after blocking the defects in a defective membrane that was selected for the experiments. For instance, at 263 K and 9 bar feed pressure, the CO 2 /H 2 separation factor increased from 8.5 to 36 after modification of the membrane, whereas the CO 2 flux only decreased by ca. 40%. [ABSTRACT FROM AUTHOR]

Abstract: Separation of carbon dioxide and methane is an important issue in upgrading low-quality natural gas. Adsorption equilibria and kinetics of CO2 and CH4 on a copper metal–organic framework (MOF), Cu(hfipbb)(H2hfipbb)0.5 [H2hfipbb=4,4′-(hexafluoroisopropylidene) bis(benzoic acid)], were investigated to evaluate the feasibility of removing CO2 from CH4 in a pressure swing adsorption process using this new MOF adsorbent. The heat of adsorption of CO2 on the Cu–MOF at zero-coverage (29.7kJ/mol) is much lower than those on a carbon molecular sieve and a zeolite 5A adsorbent; and the heat of adsorption of CH4 on the Cu–MOF (21.4kJ/mol) is similar to that on the zeolite 5A adsorbent and smaller than that on a carbon molecular sieve. The Cu–MOF being investigated has apertures of (∼3.5×3.5Å), which favors the kinetically controlled separation of CO2 and CH4. The kinetic selectivity is found to be 26 at 298K, and the overall selectivity (combining the equilibrium and kinetic effects) is about 25 for an adsorption separation process. These results suggest that the Cu–MOF adsorbent is an attractive alternative adsorbent for the CO2/CH4 separation. [Copyright &y& Elsevier]

Postural orthostatic tachycardia syndrome (POTS) typically occurs in youths, and early accurate POTS diagnosis is challenging. A recent hypothesis suggests that upright cognitive impairment in POTS occurs because reduced cerebral blood flow velocity (CBFV) and cerebrovascular response to carbon dioxide (CO2) are nonlinear during transient changes in end-tidal CO2 (PETCO2). This novel study aimed to reveal the interaction between cerebral autoregulation and ventilatory control in POTS patients by using tilt table and hyperventilation to alter the CO2 tension between 10 and 30 mmHg. The cerebral blood flow velocity (CBFV), partial pressure of end-tidal carbon dioxide (PETCO2), and other cardiopulmonary signals were recorded for POTS patients and two healthy groups including those aged >45 years (Healthy-Elder) and aged <45 years (Healthy-Youth) throughout the experiment. Two nonlinear regression functions, Models I and II, were applied to evaluate their CBFV-PETCO2 relationship and cerebral vasomotor reactivity (CVMR). Among the estimated parameters, the curve-fitting Model I for CBFV and CVMR responses to CO2 for POTS patients demonstrated an observable dissimilarity in CBFVmax (p = 0.011), mid-PETCO2 (p = 0.013), and PETCO2 range (p = 0.023) compared with those of Healthy-Youth and in CBFVmax (p = 0.015) and CVMRmax compared with those of Healthy-Elder. With curve-fitting Model II for POTS patients, the fit parameters of curvilinear (p = 0.036) and PETCO2 level (p = 0.033) displayed significant difference in comparison with Healthy-Youth parameters; range of change (p = 0.042), PETCO2 level, and CBFVmax also displayed a significant difference in comparison with Healthy-Elder parameters. The results of this study contribute toward developing an early accurate diagnosis of impaired CBFV responses to CO2 for POTS patients. [ABSTRACT FROM AUTHOR]