Gas Transport Pathways Research Articles

Enhanced CO2 separation properties by incorporating poly(ethylene glycol)-containing polymeric submicrospheres into polyimide membrane

Poly(ethylene glycol)-containing polymeric submicrospheres (PEGSS) were synthesized via distillation precipitation polymerization and incorporated into polyimide (PI) matrix to prepare hybrid membranes. The PEGSS and hybrid membranes were characterized by Fourier transform infrared (FTIR) spectra, transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD) and differential scanning calorimetry (DSC). Single gas permeabilities of N2, CH4 and CO2 were measured by using the time-lag method. The hybrid membranes showed enhanced CO2 permeability and CO2/N2, CO2/CH4 selectivities at low PEGSS loadings. The favorable affinity between PEGSS and CO2 greatly enhanced CO2 solubility and thus CO2 permeability. Whereas, N2 and CH4 permeabilities both decreased for the tortuous gas transport pathways by PEGSS incorporation. Particularly, PI–PEGSS(20) membrane, with 20wt% PEGSS loading, showed 35% of increase in CO2 permeability and 104% of increase in CO2/N2 selectivity compared with those of pristine polyimide membrane.

Feasibility of Mixed Matrix Membrane Gas Separations Employing Porous Organic Cages

Porous additives offer an attractive pathway to enhance the performance of polymeric gas separation membranes. Previously reported porous additives, such as zeolites and metal–organic frameworks, suffer from poor interfacial binding with the polymer matrix, which leads to nonselective gas transport pathways. Porous organic cages (POCs) are an exciting new family of soluble additives that could ameliorate these transport issues by integrating intimately with the polymer matrix. By using Voronoi network analysis, grand canonical Monte Carlo simulations, and molecular dynamics, we provide a theoretical assessment of the benefit of using POCs as additives for mixed matrix membranes (MMMs). We reveal that some MMMs containing POCs exhibit enhanced selectivity and permeability compared to the neat polymer matrix, particularly for H2/CO2 separations.

Effective-Diffusivity Measurement of Partially-Saturated Fuel-Cell Gas-Diffusion Layers

In proton-exchange-membrane fuel cells, flooding of the cathode gas-diffusion layer (GDL) hinders the gaseous reactant transport and thereby limits cell performance. The understanding of the effective diffusivity of the reactants through the GDL is essential for performance optimization and material design. In this paper, the effective diffusivities of unsaturated and partially-saturated GDLs are experimentally examined using an ex-situ electrochemical limiting-current method for various, uncompressed GDLs including different PTFE loadings. For unsaturated (including PTFE loadings) and partially-saturated (no PTFE) GDLs, the experimental results follow a power law with respect to porosity and saturation, respectively. PTFE treatment favorably changes the liquid distribution for improved gas-transport pathways, and a new correlation is proposed using a cumulative log-normal distribution function; however, the impact of PTFE on the overall effective diffusivity depends on the specific GDL structure. This work provides insights for fuel-cell models and transport phenomena, which can lead to the optimal GDL material design and cell operation.

Simulation of Gas Transfer at the Scale of a Disposal Cell in the Context of High-Level and Long-Lived Radwaste Disposal in a Deep Clay Formation During the Operating Phase

The choice of the Callovo-Oxfordian formation in eastern France for construction of a proposed repository for high-level, long-lived radioactive waste (HLW) is based primarily on the low hydraulic conductivity of the clay-rich host rock. This property is also intrinsically linked to a low capacity of the rock to evacuate the significant amounts of hydrogen gas generated over time by processes such as anoxic corrosion of metallic materials and radiolysis of organic waste. The effects of hydrogen production on the behavior and safety performance of the disposal system components must be evaluated for the operating and postclosure periods of the repository. In order to do this, numerical simulations using TOUGH2-MP were performed on a vitrified waste (HLW) disposal cell and its access drift, for the operating period. The objective was to investigate generation and transfer of hydrogen within and outside the disposal cell, coupled with the desaturation of the access drift near field due to the combined action of drift ventilation and the coupled behavior of dry air and hydrogen within the disposal cell. Particular attention was focused on the form of hydrogen (expressed or dissolved), total gas pressure buildup, degree of gas saturation, gas transport pathways, gas concentrations, and gas exchanges between the disposal cell and the access drift.Simulation results show the validity of the conceptual assumption based on anoxic conditions in the useful part of the disposal system. The major part of the hydrogen comes to the access drift during the operating phase. Internal boundaries between interface zones and concrete lining are preferential pathways for the gas transfer.

The impact of hardpans and cemented layers on oxygen diffusivity in mining waste heaps: Diffusion experiments and modelling studies

This study reports column tests and modelling results to assess the impact of hardpans and cemented layers on oxygen supply in mine waste sediments. The analysed sediment samples were obtained from a low-sulphide and low-carbonate polymetallic mine waste tailings impoundment located in the Freiberg mining district in Germany. The three samples were characterised by different degrees and types of cementation. After physical and mineralogical properties of the samples had been determined, breakthrough curves of oxygen were measured in column studies at different degrees of water saturation, and the diffusivities were assessed using a numerical modelling approach. Results demonstrate that cemented layers and hardpans in undisturbed sediments associated with fine-grained material operate as preferential pathways for diffusive gas transport during rewetting, leading to higher oxygen diffusivities compared to disturbed sediments. Under air-dry conditions, the disturbed samples show higher diffusivities than the undisturbed sample, indicating clogging of the porosity by precipitation of secondary minerals such as trivalent Fe oxyhydroxides acting as a barrier and thereby decreasing the diffusivity of the undisturbed sample. In contrast to sediments without cementation, diffusion experiments of sediments with cemented layers used in this study yield similar tortuosities in spite of their different grain size distributions, pointing to the important role of these heterogeneities for gas diffusion.

Effects of regional-scale and convective transports on tropospheric ozone chemistry revealed by aircraft observations during the wet season of the AMMA campaign

Abstract. The African Monsoon Multidisciplinary Analyses (AMMA) fourth airborne campaign was conducted in July–August 2006 to study the chemical composition of the middle and upper troposphere in West Africa with the major objective to better understand the processing of chemical emissions by the West African Monsoon (WAM) and its associated regional-scale and vertical transports. In particular, the french airborne experiment was organized around two goals. The first was to characterize the impact of Mesoscale Convective Systems (MCSs) on the ozone budget in the upper troposphere and the evolution of the chemical composition of these convective plumes as they move westward toward the Atlantic Ocean. The second objective was to discriminate the impact of remote sources of pollution over West Africa, including transport from the middle east, Europe, Asia and from southern hemispheric fires. Observations of O3, CO, NOx, H2O and hydroperoxide above West Africa along repeated meridional transects were coupled with transport analysis based on the FLEXPART lagrangian model. The cross analysis of trace gas concentrations and transport pathways revealed 5 types of air masses: convective uplift of industrial and urban emissions, convective uplift of biogenic emissions, slow advection from Cotonou polluted plumes near the coast, meridional transport of upper tropospheric air from the subtropical barrier region, and meridional transport of Southern Hemisphere (SH) biomass burning emissions. O3/CO correlation plots and the correlation plots of H2O2 with a OH proxy revealed not only a control of the trace gas variability by transport processes but also significant photochemical reactivity in the mid- and upper troposphere. The study of four MCSs outflow showed contrasted chemical composition and air mass origins depending on the MCSs lifetime and latitudinal position. Favorables conditions for ozone production were found for MCSs with increased MCS lifetime (>1.5 days), which allowed for more H2O2 formation, and with trajectories crossing the 10° N latitude, which increased CO transport to the upper troposphere. The upper tropospheric concentrations sampled in the MCS outflow regions showed mixed origins including local vertical convective transport, and uplifting of air from the low troposphere over the middle-east related to the summer Asian low pressure system or from the southern hemispheric fires.

Microscale mechanisms of gas exchange in fruit tissue

* Gas-filled intercellular spaces are considered the predominant pathways for gas transport through bulky plant organs such as fruit. Here, we introduce a methodology that combines a geometrical model of the tissue microstructure with mathematical equations to describe gas exchange mechanisms involved in fruit respiration. * Pear (Pyrus communis) was chosen as a model system. The two-dimensional microstructure of cortex tissue was modelled based on light microscopy images. The transport of O(2) and CO(2) in the intercellular space, cell wall network and cytoplasm was modelled using diffusion laws, irreversible thermodynamics and enzyme kinetics. * In silico analysis showed that O(2) transport mainly occurred through intercellular spaces and less through the intracellular liquid, while CO(2) was transported at equal rates in both phases. Simulations indicated that biological variation of the apparent diffusivity appears to be caused by the random distribution of cells and intercellular spaces in tissue. Temperature does not affect modelled gas exchange properties; it rather acts on the respiration metabolism. * This modelling approach provides, for the first time, detailed information about gas exchange mechanisms at the microscopic scale in bulky plant organs, such as fruit, and can be used to study conditions of anoxia.

Optimization of the Cathode Catalyst Layer Composition using a Novel 2-step Preparation Method

For optimal performance and high durability PEM fuel cells run at high water saturation levels. Excess liquid water can block pores in both the gas diffusion layer and the catalyst layer and reactant gasses can't access the active catalyst sites. Thus, to prevent electrode flooding the catalyst layer structure has to provide pathways for both gas and liquid water transport as well as high ionic and electronic conductivity. This study presents a 2-step preparation method for such a catalyst layer and also gives an optimal composition of the catalyst ink resulting in high fuel cell performance. Teflon® has been incorporated into the catalyst layer to provide a hydrophobic pathway without blocking active catalyst sites. In general, less Nafion® content needed more Teflon® in the catalyst ink. The optimal ratio was found to be C:Nafion®:Teflon® = 1:0.875:0.875.

CO2 sorption and diffusion in polymethyl methacrylate-clay nanocomposites

This study reports the glass transition temperature (Tg), and sorption and diffusion of subcritical CO2 gas in polymethyl methacrylate (PMMA) nanocomposites containing organically modified smectite clay, Cloisite 20A (C20A). A range of methods for preparing the PMMA-clay nanocomposites was investigated and a solution coprecipitation method was selected as the most appropriate. Using this method, PMMA nanocomposite containing 2, 4, 6, and 10 wt% nanoclay loadings were prepared. Wide-angle X-ray diffraction (XRD) analysis and scanning electron microscopy (SEM) indicated that the 2 wt% nanocomposite materials had a well-dispersed intercalated clay structure. The Tg for PMMA-C20A nanocomposites, as measured by differential scanning calorimetry (DSC), was found to be independent of the clay loading. CO2 solubility studies from 0 to 65°C and pressures up to 5.5 MPa using an in situ gravimetric technique were performed on compression-molded films. The organoclay was found to have no effect on the solubility of CO2 in PMMA, and therefore the solubility of CO2 in the nanocomposite can be determined from the solubility of CO2 in the matrix polymer alone. Diffusion coefficients were determined using the appropriate transport models for these test conditions and the diffusion coefficients for CO2 in PMMA-C20A composites were found to increase with organoclay loading. It is believed that the processing path taken to prepare the nanocomposites may have resulted in the agglomeration of the C20A organoclay, thereby preventing the polymer chains from fully wetting and intercalating a large number of clay particles. These agglomerations are responsible for the formation of large-scale holes within the glassy nanocomposite, which behave as low resistance pathways for gas transport within the PMMA matrix. POLYM. ENG. SCI., 45:904–914, 2005. © 2005 Society of Plastics Engineers

The Role of Water in Gas Hydrate Dissociation

When raised to temperatures above the ice melting point, gas hydrates release their gas in well-defined, reproducible events that occur within self-maintained temperature ranges slightly below the ice point. This behavior is observed for structure I (carbon dioxide, methane) and structure II gas hydrates (methane−ethane, and propane), including those formed with either H2O- or D2O-host frameworks, and dissociated at either ambient or elevated pressure conditions. We hypothesize that at temperatures above the H2O (or D2O) melting point: (1) hydrate dissociation produces water + gas instead of ice + gas, (2) the endothermic dissociation reaction lowers the temperature of the sample, causing the water product to freeze, (3) this phase transition buffers the sample temperatures within a narrow temperature range just below the ice point until dissociation goes to completion, and (4) the temperature depression below the pure ice melting point correlates with the average rate of dissociation and arises from solution of the hydrate-forming gas, released by dissociation, in the water phase at elevated concentrations. In addition, for hydrate that is partially dissociated to ice + gas at lower temperatures and then heated to temperatures above the ice point, all remaining hydrate dissociates to gas + liquid water as existing barriers to dissociation disappear. The enhanced dissociation rates at warmer temperatures are probably associated with faster gas transport pathways arising from the formation of water product.

A probabilistic model and software tool for evaluating the long-term performance of landfill covers

A probabilistic model and software tool has been developed to assist in the selection, design, and monitoring of long-term covers for contaminated subsurface sites. The software platform contains multiple modules that can be used to simulate relevant features, events, and processes that include water flux through the cover, source-term release, vadose-zone transport, saturated-zone transport, gas transport, and exposure pathways. The component models are integrated into a probabilistic total-system performance-assessment model within a drag-and-drop software platform. Uncertainty and sensitivity analyses can be conducted that yield the following primary benefits: (1) quantification of uncertainty in the simulated performance metrics; (2) identification of parameters most important to performance; and (3) comparison of alternative designs to optimize cost and performance. A case study has been performed using the Monticello Mill Tailings Site in Utah to illustrate the important features and benefits of the modeling approach and software.

Evaluation of Gas Transport Properties of Backfill Materials for Waste Disposal: H2 Migration Experiments in Compacted Fo-Ca Clay

AbstractSwelling clays may play a major role in the underground disposal of high-level nuclear waste (HLW) in deep geological formations. A multibarrier concept including a waste container, a steel overpack, an engineered barrier consisting of compacted clay (buffer) and the host rock is a potential technique for such HLW disposal. It is anticipated that H2 will be the main gas generated, mainly by anaerobic metal corrosion during disposal lifetime. After complete resaturation, the clay barrier will have a very low permeability and H2 may accumulate in the space between the overpack and the clay barrier. This could result in pressures exceeding the resistance of the clay and damage to the entire engineered barrier system (EBS).The French Atomic Energy Commission (CEA) has performed an experimental program on H2 gas migration in a French clay referenced Fo-Ca. The tests were conducted with a specific odometer-type cell developed by the Power Reactor and Nuclear Fuel Corporation (PNC) in Japan. Permeability tests on compacted Fo-Ca clay samples of specific dry densities between 1.6 and 1.9 and for water saturation degrees between 70 and 100% provided significant H2 permeability data (ranging from 10−15 to 10−21 m2). Gas migration experiments were also performed to study the behavior of Fo-Ca clay under high gas pressure. Two kinds of gas transport threshold pressures were detected in unsaturated compacted clay. The first one, called “critical pressure”, is the pressure over which a gas outflow migrates into the clay. This pressure probably exceeds the capillary pressure of the largest pores within the clay. The second one, called “breakthrough pressure”, is detected with increasing gas injection pressure. A sudden rise of gas outflow is subsequently observed. The occurrence of the gas breakthrough is associated with the aperture and propagation of preferential gas transport pathways in the clay. This is apparently linked to the hydromechanical properties and stress state of the clay. When clay samples are not fully watersaturated, the critical pressure is always lower than the breakthrough pressure. However, when the clay is saturated, the 2 pressures appear to be very close. Finally, this study showed that breakthrough pressure—a key parameter for the long-term stability of the clay buffer—varies considerably with the gas injection increments and with the size of the sample. The results of these experiments are consistent with those reported previously by other investigators.

Eleocharis sphacelata: internal gas transport pathways and modelling of aeration by pressurized flow and diffusion.

The ability of diffusive gas transport and pressurized, convective flow to satisfy internal oxygen demands was examined for an aquatic sedge, Eleocharis sphacelata R. Br. Resistances to convection and diffusion through the plant were quantified from anatomical studies of the airspace dimensions, and these were used in mathematical models to calculate the fluxes required to satisfy oxygen demands measured in the tissue. The greatest resistance to diffusion in the underwater tissue was the submerged culm between the waterline and sediment surface (1560 Ms m-3 per m culm length). Resistances of the nodal intercalary meristem (52 Ms m-3 ) and rhizome internode (34 Ms m-3 ) were minor. In contrast, resistances to convection were low in the culms (38 MPa s m-3 per m culm length), and higher in the nodal meristems (93 MPa s m-3 ). The rhizome internodes had large cortical canals with a low convective resistance (0.75 MPa s m-3 ), and a parallel spongy pith with a very high resistance (518 MPa s m-3 ) that is probably short-circuited by convection. The resistance of the submerged culm means that diffusion is inadequate to satisfy oxygen demands in plants growing in >10 cm of water, and that convection is therefore essential in the natural habitat of this species (water to c. 2 m depth). However, a convective oxygen influx as low as 28 × 10-8 mol s-1 per culm (equivalent to a gas flow rate of 3 μl s-1 per culm) could satisfy the entire oxygen demand of the underwater tissue; this value is well below actual rates. At this flow rate, the spongy pith in the rhizome would also remain aerobic: it has a low resistance to diffusion (73 Ms m-3 ) and could receive sufficient oxygen by diffusion from the node. The data agree well with previous empirical measurements of convection in this species and show that diffusion and convection are both important processes for its aeration.

Senescence, and phytotoxin, insect, fungal and mechanical damage: factors reducing convective gas-flows in Phragmites australis

Premature senescence, insect bore holes and callus development in the gas-transport pathways were each shown to impair convective (pressurised) gas-flows in Phragmites australis (Cav.) Trin. ex Steudel. Premature senescence resulting from aphid and fungal attack, and possibly from the uptake of soil-borne phytotoxins, was found on average to have reduced the gas-flow potential of living culms by at least 70%. This was attributed chiefly to a reduction in humidity-induced convection resulting from a loss of healthy leaf-sheath area. Insect bore holes, found in living culms, reduced potential humidity-induced convective flows by varying amounts, by causing the culms to be leaky. It was shown that the degree of leakiness of culms should be dependent upon the position of the bore holes along the culm. Irrespective of their position and the wind speed, bore holes in old dead culms reduced Venturi-induced convection to almost zero. However, such holes could also increase convective flows by reducing resistance to venting from the underground parts, basal holes conferring lower resistance than more apical ones. In addition, it was concluded that callus development within gas pathways of the plant is stimulated by insect damage, by mechanical damage/flooding and by uptake of phytotoxins such as acetic acid and sulphide, and that extensive callus production might completely impede all pressurised gas flows. It is suggested that each of the above factors has the potential to contribute to Phragmites die-back by adversely affecting rhizome, root and rhizosphere aeration.

Phragmites australis– A preliminary study of soil‐oxidizing sites and internal gas transport pathways

SUMMARYThe possible pathways of oxygen transport within Phragmites australis and out into the soil were investigated by means of pressurized air flow, methylene‐blue dye and by anatomical studies.In most cases, gas was easily forced through the plants, thus demonstrating gas‐space connexion between aerial shoot, rhizomes and roots and a relatively low resistance to flow. Gas‐space connexions between the pith cavity and cortical aerenchyma in the rhizomes were also detected and were investigated anatomically they are present only in nodal regions just above the insertion of the pith diaphragm and take the form of radial channels through the stelar cylinder. Similar pathways in the culm connect the pith cavity with nodal lines of stomata and it is suggested that these regions may often form the major oxygen entry point to the rhizome‐root system.A failure to effect pressurized gas‐flow through some rhizomes led to the detection of secondary barriers to longitudinal gas‐flow. Seemingly a response to wounding, these are an impenetrable barrier to gases, to flooding, and probably also to microorganisms, and take the form of tylosoidal growths from and within regions of stellate‐and/or circumvascular‐parenchymas such as those of the nodal diaphragms. Stellate parenchymas of various sorts were found to be a common feature of the gas‐path in Phragmites and evidently they form an important component of the gas‐space system occurring within the nodal radial channels, the nodal diaghragms and, rather unusually, the root‐shoot junctions where they must confer a porosity which is higher than for most plants.As measured by methylene‐blue oxidation, oxygen release from underground parts was most rapid from young adventitious and secondary roots and particularly basal tufts of fine laterals. Zones of oxidation also occurred around the sprouting tips of horizontal and vertical rhizomes. Dormant tips showed very little oxidizing power, whilst the rhizomes themselves and the older parts of adventitious roots showed none. This was associated with cuticularization of the rhizome and hypodermal suberization and lignification; oxygen‐impermeable regions of the roots were also hyperdermally suberized and lignified.The findings are discussed in relation to the use of Phragmites in sewage‐treatment beds.