Tensiometers are commonly used for measuring soil water matric pressures. Unfortunately, the water-filled reservoir of conventional tensiometers limits their applicability to soil water matric pressures above approximately 20.085 MPa. Tensiometers filled with a polymer solution instead of water are able to measure a larger range of soil water matric pressures. We designed and constructed six prototype polymer tensiometers (previously called osmotic tensiometers) consisting of a wide-range pressure transducer with a temperature sensor, a stainless steel casing, and a ceramic plate with a membrane preventing polymer leakage. A polymer chamber (0.1-2.2 cm 3 ) was located between the pressure transducer and the plate. We tested the polymer tensiometers for long-term operation, the effects of temperature, response times, and performance in a repacked sandy loam under laboratory conditions. Several months of continuous operation caused a gradual drop in the osmotic pressure, for which we developed a suitable correction. The osmotic potential of polymer solutions is temperature dependent, and requires calibration before installation. The response times to sudden and gradual changes in ambient temperature were found to be affected by polymer chamber height and polymer type. Practically useful response times (,0.2 d) are feasible, particularly for chambers shorter than 0.20 cm. We demonstrated the ability of the instrument to measure the range of soil water pressures in which plant roots are able to take up water (from 0 to 21.6 MPa), to regain pressure without user interference and to function properly for time periods of up to 1 yr.
Single gas adsorption isotherms of methane and carbon dioxide on micro-porous Norit RB1 activated carbon were determined in a gravimetric analyser in the temperature range of 292 to 349 K and pressures to 0.8 Mpa. Furthermore binary isotherms of carbon dioxide and methane mixtures were determined at 292 K and pressures up to 0.65 MPa. Adsorbed phase compositions were determined from the gravimetric data by the rigorous thermodynamic method of Van Ness.These experimental binary equilibrium data were compared with equilibrium data calculated by the Ideal Adsorbed Solution (IAS) model. Only moderate agreement could be obtained.Finally, activity coefficients, accounting for the non-ideality of the adsorbate mixture, were calculated from the experimental data. The Wilson equation, derived for bulk solutions, was fitted on these activity data and the Wilson interaction parameters were determined. The Wilson equation proved to correlate the experimental data reasonably. However, the Wilson interaction parameters are not only completely different from those found for bulk solutions, but also the physical interpretation of these parameter values is completely lacking.It is concluded that new solution models should be developed encompassing both non-ideal solution behaviour and surface heterogeneity.
Inorganic microporous materials show great potential for applications in industrial catalysis, separation technology, membranes, sensors, and optical devices. The most common amorphous, microporous inorganic material is silica. Due to its small pores and straightforward synthesis, amorphous silica has been considered as a promising material for membrane applications. However, its moderate hydrothermal stability limits its extended application in harsh environments.[1] Of particular interest as alternative materials for stable, high-performance membranes are sol-gel-derived amorphous oxides of transition metals, such as titania and zirconia. These materials show superior stability compared to silica; [2] however, their synthesis is complicated by the high reactivity of the required precursors.[3] To date, a few groups have been able to prepare amorphous titania and zirconia membranes for aqueous nanofiltration.[4]Herein, we describe the preparation and characterization of microporous zirconia-titania composite membranes. Preparation has been carried out via two different synthesis routes. The first route is based upon using the diethanolamine-stabilized heterometallic precursor Zr{l-g 3 -NH(C 2 H 4 O) 2 } 3 [Ti-(O i Pr) 3 ] 2 (1).[5] The second route involves synthesis using 2, a mixture of zirconium n-propoxide, titanium n-propoxide, and diethanolamine in a molar ratio equivalent to that in 1. Figure 1a shows thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) data for uncalcined powders derived from either 1 or 2. The data are similar for both powders. A sharp initial weight loss is observed with an onset at 50°C, corresponding to the removal of adsorbed solvent molecules. A gradual decrease in weight is subsequently observed up to 300°C. This is attributed to the removal of ligands from the outer surface of the material. Mass spectrometry confirmed the presence of nitrogen in the sweep gas, originating from the amine groups of the ligands. In the range ∼ 300-350°C, a sharp decrease in weight is observed, which is accompanied by a large change in the heat flow centered at 350°C. This can be attributed to the removal of ligands located inside the material. Mass spectrometry again confirmed a high concentration of nitrogen in the sweep gas. A further increase of temperature to 450-600°C causes removal of the remaining alkoxide ligands.The DSC data suggest that crystallization occurs at ∼ 750°C, as evidenced by the sharp exothermic peak at this COMMUNICATIONS
Methylated microporous silica with high thermal stability and tuneable hydrophobicity was obtained by acid-catalysed sol-gel hydrolysis and condensation of mixtures of tetraethylorthosilicate (TEOS) and methyltriethoxysilane (MTES). The gels exhibited a trend towards smaller ultramicropores with increasing methyl content, while in addition some supermicropores were formed with sizes of around 2 nm. For low MTES concentration, dilution prior to gelation and ageing resulted in materials with clearly smaller ultramicropores, whereas only a minor effect of dilution on structure was found at high MTES concentration. The small ultramicropore size in 'diluted' materials can be associated with a higher extent of condensation of mainly TEOS monomers. Stable structures formed from MTES in an early stage of synthesis may explain the particular micropore structure of MTES-rich gels. With increasing methyl content and with dilution of the sol, the affinity of the surface to water was strongly decreased. The applicability of microporous silica in wet atmospheres may thus be improved by methylation, and their pore structure modified by adaptation of the recipe, which would be highly relevant for industrial gas and liquid separation by inorganic membranes.
a b s t r a c tHybrid silica membranes have demonstrated to possess a remarkable hydrothermal stability in pervaporation and gas separation processes allowing them to be used in industrial applications. In several publications the hydrothermal stability of pure silica or that of hybrid silica membranes are investigated. To gain deeper insight into the mechanism of hydrothermal stability of silica-based membranes we report a comparison under identical conditions of the gas permeation performance of silica (TEOS), hybrid silica (BTESE) and Zr-doped BTESE (Zr-BTESE) membranes before and after hydrothermal treatments. First, a fast and straightforward hydrothermal stability test at 100°C was applied to screen these membranes. The BTESE and Zr-BTESE membranes maintained their excellent performance after this test, though the TEOS membranes lost their selectivity. Second, hydrothermal tests under water gas shift (WGS) conditions were performed at different temperatures. No significant changes in permeance and selectivity were observed for BTESE derived membranes after a hydrothermal treatment at 300°C. Surprisingly, a large reduction in carbon dioxide permeance was observed for Zr-BTESE hybrid silica membranes after a hydrothermal treatment at 200 or 300°C, resulting in a significant increase of the H 2 /CO 2 permselectivity from 12 to 35.
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