Ammonia Synthesis at Low Pressure

Ammonia can be synthesized at low pressure by the use of an ammonia selective absorbent. The process can be driven with wind energy, available locally in ...

University of Minnesota – Twin Cities

A poly(vinyl alcohol) (PVA) membrane containing iron (Fe(0)) particles was developed and tested as a model barrier for contaminant containment. Carbon tetrachloride, copper (Cu2+), nitrobenzene, 4-nitroacetophenone, and chromate (Cr04(2-)) were selected as model contaminants. Compared with a pure PVA membrane, the Fe(0)/PVA membrane can increase the breakthrough lag time for Cu2+ and carbon tetrachloride by more than 100-fold. The increase in the lag time was smaller for nitrobenzene and 4-nitroacetophenone, which stoichiometrically require more iron and for which the PVA membrane has a higher permeability. The effect of Fe(0) was even smaller for CrO4(2-) because of its slow reaction. Forty-five percent of the iron, based on the content in the dry membrane prior to hydration, was consumed by reaction with Cu2+ and 15% by reaction with carbon tetrachloride. Similarly, 25%, 17%, and 6% of the iron was consumed by nitrobenzene, 4-nitroacetophenone, and CrO4(2-), respectively. These percentages approximately double when the loss of iron during membrane hydration is considered. The permeability of the Fe(0)/PVA membrane after breakthrough was within a factor of 3 for that of pure PVA, consistent with theory. These results suggest that polymer membranes with embedded Fe(0) have potential as practical contaminant barriers.

A polymer membrane containing Fe(0) as a contaminant barrier.

In diaphragm cell experiments, a permeable model reactive barrier for the containment of cesium is tested. Primary targets for cesium containment are former plutonium processing sites (e.g., Hanford, WA and Savannah River, SC), which are currently contaminated with cesium-137. Adding up to 10 wt % crystalline silicotitanate, a sacrificial reagent, to poly(vinyl alcohol) films increases the time before cesium can cross the film by a factor of 30. The increased lag times are consistent with theories developed for this type of reactive membrane. Theory also correctly predicts the effects of cesium concentration and membrane thickness on membrane performance. Because the relative improvements of the model barrier are expected to be independent of the polymer used, these increased lags should hold for less permeable polymers that are more resistant to radiation, although these polymers have not been tested.

Permeable membranes containing crystalline silicotitanate as model barriers for cesium ion.

Breakthrough across high-density polyethylene (HDPE) was measured for 2,3',4',5-tetrachlorobiphenyl and a higher-solubility surrogate, 1,2,4-trichlorobenzene. Addition of powdered activated carbon (0.14 g carbon/cm(3) membrane) reduced pseudo-steady-state flux through thin HDPE membranes by approximately 60%. Breakthrough curves for activated carbon-containing membranes were best described by a model in which sorption to the carbon was limited by the rate of diffusion from the bulk membrane to the carbon particle surfaces. Field-scale estimates based on this model show a substantial (over 10 orders of magnitude) reduction in flux for the activated carbon-containing HDPE compared with pure HDPE. The flux of 2,3',4',5-tetrachlorobiphenyl through a composite membrane with thin layers of poly(vinyl alcohol) (PVA) with 0.05 g carbon/cm(3) and pure HDPE was 69% lower than expected for a similar layered membrane without the sorptive scavenger. This flux reduction was achieved with less than a third of the carbon used in the HDPE case, an improvement that is likely the result of better solute uptake in the hydrophilic PVA layer.

Geomembranes containing powdered activated carbon have the potential to improve containment of chlorinated aromatic contaminants.

We report gas and liquid transport measurements through membranes that have 40% voids made of 14 nm pores. A reactive polylactide-polynorbornenylethylstyrene block polymer is used as a structural template in the polymerization of dicyclopentadiene during the membrane formation process. After the membrane is cast, the pore structure is formed by etching the polylactide component using dilute aqueous base. The pore structure is isotropic; therefore, there is no need for special alignment techniques. Knudsen diffusion experiments and water flow experiments show pores with a tortuosity of 1.81 and a size of 14 nm, a diameter consistent with nitrogen adsorption and small-angle X-ray scattering measurements. These membranes are effective for ultrafiltration, with molecular weight cutoffs (MWCO) consistent with theoretical predictions with no adjustable parameters. These MWCO's can be tuned by changing the size of the constituent blocks in the templating copolymer.

Diffusion and flow across nanoporous polydicyclopentadiene-based membranes.

Ultrafiltration membranes with an 80 nm thick block polymer derived selective layer containing 20 nm cylindrical pores were prepared by removing poly(lactide) (PLA) from a poly(styrene)-b-poly(isoprene)-b-poly(lactide) (PS-PI-PLA) film onto a microporous polymer support. The block polymer film adopted a core(PLA)-shell(PI) cylindrical morphology in which vertically-oriented PLA cylinders were degraded to leave PI-lined channels in a PS matrix. Thanks to the combination of PS and PI in the nanoporous matrix, chemical cross-linking was not needed to provide mechanical stability in the thin film. The membranes showed a hydraulic flux of 165 L m(-2) h(-1) bar(-1) and were able to size-discriminate poly(ethylene oxide) (PEO) solutes in agreement with theoretical predictions.

Edward Cussler

Department of Chemical Engineering and Materials Science

University of Minnesota – Twin Cities

Ammonia Synthesis at Low Pressure

Edward Cussler

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University of Minnesota – Twin Cities

Ammonia Synthesis at Low Pressure

Department of Chemical Engineering and Materials Science