Low-permeability Materials Research Articles

Simulation of the Wicking Effect in a Two-Layer Soil Liner System

A series of simulation runs were made using the computer model VSAFT2 (Yeh & Srivastava 1990) to estimate the "wicking" potential of five low-permeability liner materials including excel, celite slurry, kiln dust, rock creek clay and celite waste. The unsaturated hydraulic properties for these materials were assumed to be represented by the van Genuchten model (1978). The hypothetical liner system simulated consisted of 0.305 m (1 foot) of low permeability material underlain by a medium sand. The purpose of this modelling effort was to quantify the effects of liner thickness and leakage rate on the lateral spreading of the wetting front, and to determine the effectiveness of the diffusivity function in predicting the migration of the wetting front. Results indicate, as expected, that the liner material with the lowest permeability retards vertical movement the most; however, the horizontal wicking effect is also limited. Additional multi-layer liner systems will be simulated in the next phase of the project to improve liner wicking performance.

Analytical solution to two-dimensional axisymmetric gas flow with Klinkenberg effect

The gas-slippage phenomenon of gas permeability, commonly known as the Klinkenberg effect, has been neglected in the analysis of gas flow. In this work, an exact analytical solution to the gas flow in a two-dimensional axisymmetric-flow domain with anisotropic gas permeability that includes the Klinkenberg effect is presented. The solution is also extended to provide an analytical expression for the stream function. The results suggest that the Klinkenberg effect is important with low-permeable materials such as clay and silt. The errors involved in neglecting the Klinkenberg effect are highest at the well and are reduced with the radial distance.

Preparative Method and Analysis by OM, SEM and EPMA of Porous, Brittle and Low Permeability Rocks and Materials: The Case of Pumices

A method is reported for the preparation of flat specimens (30 μm thick) of porous, brittle and low permeability materials and rocks for both optical microscopy (OM), scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). The method, based on alternating embeddings in vacuum with epoxy resin and grindings with final polishing down to 0.25 μm alumina grain size, prevents the breaking of the glassy interconnections and fills with resin the majority of the bubbles and voids. It has been applied to the analysis of pumices sampled at Linosa, Sicily, which are glassy materials with inhomogeneities on a micro-scale and with a vesicular texture often not interconnected. The method has been successfully applied also to the preparation of various sedimentary rocks, synthetics, bones and biomaterials.

High-efficiency, high-density MHz magnetic components for low profile converters

A highly efficient (99.5%) transformer and resonant inductor with very high power density (1500 W/in/sup 3/) and low profile (height <0.4 in) are described. These may be the highest numbers for power density for 99.5% efficient magnetic components. The design of these components is based on a trade-off study which establishes the optimum operating frequency to be around 1 MHz. The results of this trade-off study which established the baseline designs including the sizes are described. The transformer utilizes highly efficient thin flex circuit windings. The windings are novel folded copper patterns which eliminate the need for vias and results in very low DC resistance. The windings' arrangements have a high degree of interleaving between the primary and the secondary windings, resulting in very low AC conductor losses due to skin and proximity effect. The core is a low profile design with very low core losses. The resonant inductor, instead of using low permeability material, uses highly efficient high permeability material. The desired low permeability is then achieved by putting a large number (>40) of small gaps to reduce the effect of fringing fields. The high-frequency loss measurements for such highly efficient inductors requires a special ring down measurement set-up which is also described.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Design and Testing Criteria for Bipolar Plate Materials for Pem Fuel Cell Applications

ABSTRACTBipolar plates for proton exchange membrane (PEM) fuel cells are currently under development. These plates separate individual cells of the fuel cell stack, and thus must be sufficiently strong to support clamping forces, be electrically conducting, be fitted with flow channels for stack thermal control, be of a low permeability material to separate safely hydrogen and oxygen feed streams, be corrosion resistant, and be fitted with distribution channels to transfer the feed streams over the plate surface. To date, bipolar plate costs dominate stack costs, and therefore future materials need to meet strict cost targets.A first step in the bipolar plate development program is an assessment of design constraints. Such constraints have been estimated and evaluated and are discussed here. Conclusions point to promising advanced materials, such as conductive, corrosion resistant coatings on metal substrates, as candidates for mass production of fuel cell bipolar plates. Possible candidate materials are identified, and testing procedures developed to determine suitability of various materials.

Capture Zone Theory in the Recovery of a Free Product Plume Resulting from an Underground Storage Tank Leak: ABSTRACT

Capture zone theory has been used to recover a combined free product and dissolved hydrocarbon plume from the subsurface resulting from an underground storage tank leak in an urban area. Results of hydrostratigraphic mapping demonstrate that product lost into the subsurface was trapped in sand and gravel materials in a meander bend associated with a glacial outwash channel. Approximately 20,000 gal of gasoline and diesel fuel were lost to the subsurface. The vertical and horizontal extent of the free-product plume was mapped based on the observed oil saturated thickness in soil samples. The mapping indicated that the plume was wedge shaped, with a maximum thickness of free product in soil of 3.5 ft. Hydraulic data from pump tests and slug tests indicate that product migrated into permeable sediments associated with a member bend. Low-permeability silt and clay materials surrounding the meander bend acted as a barrier preventing migration of the plume off-site. The ARMOS model and the CAPZONE model were used to determine the optimum location of product recovery wells and to evaluate the effect of boundary conditions on the safe yield of the outwash channel so that hydrocarbon recovery could be optimized over time.

The influence of surcharge level on the flowrate of bulk solids from mass flow bins

In this paper, theoretical and experimental results are presented which indicate that the influence of material surcharge level can be significant for relatively low permeability materials. Three regions of particle permeability are proposed to generalize the effect of surcharge level. For very fine powders an unsteady condition of particle flow (periodic flow) was observed and it was noted that the lowest flowrate occurring in the unsteady flow can be regarded as the maximum attainable steady flowrate when a feeder is used to diminish the flowrate fluctuation. It it found that the extent of this flowrate oscillation is material level dependent.

Fluid flow through fractures in below ground concrete vaults

Cement and concrete are used extensively to isolate waste materials from the environment and to control groundwater flow rates in mining, waste disposal, and site remediation activities. High quality concrete is a very low permeability material, however it is brittle and subject to cracking. In practice, the permeability of concrete is controlled by the fractures or cracks which form in the structures. Simplified models for flow through and around underground concrete vaults are presented and applied. Results of the applications indicate that flow rates through cracked concrete are a function of size and spacing of cracks. When cracks are small and closely spaced, resistance in passing through the cracks controls flow. However, when cracks are larger and widely spaced, resistance through the adjacent porous medium (i.e., soil) governs flow rates through cracks. If calculations consider both the adjacent porous media and the cracks, maximum predicted flow rates do not occur for the largest or smallest crack sizes and spacings, but somewhere inbetween the extremes. Further calculations illustrate that perched water will frequently be expected on top of vaults justifying use of saturated flow equations. Perched water can occur because the cracked roof of a concrete vault is analogous to capillary barriers used in some engineered cover designs. Although not a critical design concentration, results indicate that keeping the vault roof below saturation to avoid flow into the cracks is difficult and has the best chance of success for smaller, modular vaults, arid locations, and very shallow burial.

An analytical model for in situ extraction of organic vapors

This paper introduces a simple convective-flow model that can be used as a screening tool and for conducting sensitivity analyses for in situ vapor extraction of organic compounds from porous media. An assumption basic to this model was that the total mass of volatile organic chemicals (VOC) exists in three forms: as vapors, in the soil solution, and adsorbed to soil particles. The equilibrium partitioning between the vapor-liquid phase was described by Henry's law constants ( K H) and between the liquid-soil phase by soil adsorption constants ( K d) derived from soil organic carbon-water partition coefficients ( K oc). The model was used to assess the extractability of 36 VOCs from a hypothetical site. Most of the VOCs appeared to be removable from soil by this technology, although modeling results suggested that rates for the alcohols and ketones may be very slow. In general, rates for weakly adsorbed compounds ( K oc < 100 mL/g) were significantly higher when K H was greater than 10 −4 atm-m 3-mol −1. When K oc was greater than about 100 mL/g, the rates of extraction were sensitive to the amount of organic carbon present in the soil. The air permeability of the soil material ( k) was a critical factor. In situ extraction needs careful evaluation when k is less than 10 millidarcies to determine its applicability. An increase in the vacuum applied to an extraction well accelerated removal rates but the diameter of the well had little effect. However, an increase in the length of the well screen open to the contaminated zone significantly affected removal rates, especially in low-permeability materials.

Aspects of fluid flow through small flaws in membrane liners

Fluid flow through flaws (punctures, seams, or tears) in geomembranes used as flow barriers in the liners of landfills and surface impoundments is examined. Performance of a geomembrane deteriorates fairly rapidly with the first few flaws, much greater than their small area would suggest. As the membrane is penetrated with a greater number of flaws, the degradation asymptotes toward a proportionality with total flaw area. The rate of flow through the flawed membrane is frequently directly proportional to the permeability of the adjacent porous media. Geomembranes that are not used in conjunction with adjacent low-permeability porous materials are unlikely to give satisfactory performance in the event that flaws occur. A better understanding of design aspects that tend to minimize the impacts of flaws in geomembranes can lead to improved performance of landfills and surface impoundments.

Relaxation in a low-permeability porous material

Relaxation in low-permeability porous materials has been examined for nonstationary infiltration.

The Divided Flow Permeameter

AbstractIn permeameters for rigid materials such as concrete, cement paste, or rock the sample is generally sealed into the test cell with a resin or with a pressurised membrane and the permeability is calculated from the total flow through the apparatus. Clearly any leakage around the edge of the sample or other inhomogeneity in the peripheral region will lead to inaccuracy in the measured permeability. This problem is particularly severe with low permeability materials such as concrete and is further exacerbated by the fact that the true permeability of concrete can vary by many orders of magnitude and there may be little information with which to assess whether particular test results are reasonable.The divided flow permeameter offers a technique by which edge leakage and other edge effects can be assessed and several permeability results obtained from different areas of a single sample. The basic principle of this permeameter is that either the inflow or the outflow surface, or both, of the sample are divided into separate flow regions. Provided that there is no difference in pressure between the regions there should be no tendency for flow between them and the flow through each should be proportional to its area.As the technique gives several permeability results for a single sample information about permeability variation within the sample can be obtained; for example data on microcracking, moisture distribution etc. The technique can be used with gas or liquid permeants. The divided flow permeameter system developed at King's College is inexpensive and with a simple system of ‘0’ rings a flow division head can be fitted to many conventional permeameters.

Volatilization Losses of Organics During Ground Water Sampling from Low Permeability Materials

AbstractVolatilization biases that can affect a aground water sample before its collection from a monitoring well were evaluated in this study. Previous studies showed that volatilization losses during sampling of high permeability materials do not introduce unacceptable bias, except; for the most volatile compounds. In low permeability materials, however, ground water must normally accumulate for hours to days after flushing before a volume sufficient for sampling is available. During this period, the ground water sample is open to the atmosphere and volatilization can lower the concentration of volatile compounds in solution.Laboratory simulations were conducted to evaluate this bias using four chlorinated, one‐ and two‐carbon compounds. Two distinct conditions of headspace exposure were investigated: (a) the water standing in the well casing, and (b) formation water entering the screen of a well that has been dewatered during purging.Water standing in the well was depleted in volatile organics by exponential decay with a half life of about four days. Volatilization losses will be less than 10 percent if the standing time is less than about six hours. In wells that have been purged dry, volatilization losses of 10 percent are likely in as little as five minutes as the recovering formation water trickles through the headspace in the dewatered sand filter pack. Losses may reach 70 percent for recovery periods of one hour. When the sand filter pack is drained by the purging procedure, the sample should not be analyzed for volatile constituents since volatilization biases are likely to be substantial.Conventional open system monitoring wells should be used to collect volatile organic samples only if fresh formation water can be drawn into the well with minimal turbulence and exposure to the atmosphere. One should therefore avoid drawing the water level down into the sand pack when the well is purged. Specialized sampling methods should be developed and evaluated for volatile organics where sample integrity is critical.

A Novel Technique for Measurement of Tritium Permeation Through Resistant Materials Near Room Temperature: Demonstration of Method with Copper at 50–170°C

To measure tritium permeation through low-permeability materials at 50–170°C, we use highly-sensitive liquid scintillation counting to detect the permeating tritium. To validate our method, we conducted extensive experiments with copper, for which much data exists for comparison. We report permeability of tritium through copper at 50, 100, and 170°C, and discuss details of the experimental technique. Further plans are outlined.

Measurement of tritium permeation through resistant materials near room temperature

To measure tritium permeation through low-permeability materials at 50 to 170/sup 0/C, we use highly-sensitive liquid scintillation counting to detect the permeating tritium. To validate our method, we conducted extensive experiments with copper, for which much data exists for comparison. We report permeability of tritium through copper at 50, 100, and 170/sup 0/C, and discuss details of the experimental technique. Further plans are outlined. 15 refs., 5 figs., 1 tab.

Secondary Recovery of Water

Injection of air into the vadose zone has made water available for recovery by conventional means at two test sites on the High Plains of Texas. The cost of this water, exclusive of research costs, is estimated to be about $50 per acre‐ft. Large quantities of water may be available for recovery in aquifers which have experienced long term water level declines, and where a layer of low permeability material is available to prevent rapid escape of the injected air. A possible explanation of the mechanism of removal is examined, and the conclusion reached that movement occurs because of unbalanced pressure forces on the capillary water. These forces are caused by the flow of the injected air past the pore water.

Theoretical treatment of evaporation front drying

A general formalism is presented for the combined diffusion and forced flow of a binary gas mixture through a porous medium. The resulting equations are then specialized to handle the vapor region of a porous material which is drying according to a receding evaporation front model. Solutions presented for various permeabilities and temperatures give the conditions under which air is present in the pores and allow one to investigate the usefulness of the ‘boiling’ concept for the drying of low permeability materials. Evaporation front motion together with the water loss rate is examined for different curvilinear geometries both analytically and numerically.

Boiling in low-permeability porous materials

Experimental observations are reported on boiling in a vertical circular cylinder heated from below and cooled from above. The cylinder is rilled with low-permeability, water-saturated porous materials. With the onset of boiling an almost isothermal, 2-phase (wet steam) zone forms near the bottom. This zone is overlain by a liquid layer with an essentially linear vertical temperature gradient. The temperature field in the overlying liquid layer is oscillatory whenever the height of the 2-phase zone exceeds a critical value. A simple 1-dim. model is used to predict the height of the 2-phase zone, the dry-out heat flux, and a necessary condition for the formation of a 2-phase zone.

Movement of Water and Mass Transfer Into Developing Grains of Wheat

A method was developed for measuring the net fluxes of water (Fw) and dry matter (Fdm) through the stalk attaching the grain to the rachis. After the removal of the bracts, the base of the grain was embedded in plastic material of low permeability, and a capsule containing water or solutions of known water potential was sealed over the grain. Fw and Fdm were measured gravimetrically as changes in weight of the grain and the contents of the capsule during 4 days. Fdm was independent of the water potential gradient between the rachis and the atmosphere within the capsule (surrounding the grain), and was not related either to the size of Fw, or to the direction of the flow of water. Besides, Fw was too small to be consistent with a mechanism for the transport of assimilates involving proportional unloading of solute and solvent into the apoplastic phase of the grain.

Measurement of gas permeability of polymers. I. Permeabilities in constant volume/variable pressure apparatus

A constant volume/variable pressure gas permeability apparatus is described that provides accurate determination of permeabilities ranging from less than 10−1 to 106 centibarrers. Metal construction, variable temperature control, adjustable downstream pressure and constant volume, and a differential pressure transducer with automatic recording are design features that permit detailed permeability studies under a variety of environmental conditions. Pressure effects on polymer films have been investigated up to 1000 psi, and the relation of gas concentration to permeability has been studied by varying the downstream pressure and volume conditions. The high feed pressures have significantly shortened the time required to obtain meaningful data on low-permeability materials after steady-state conditions are achieved, and the variable-temperature control has permitted evaluation of temperature-related phenomena.