Preferential Channeling Research Articles

Numerical Simulations of Viscous Fingering in Fractured Porous Media

AbstractThe effect of heterogeneity induced by highly permeable fracture networks on viscous miscible fingering in porous media is examined using high-resolution numerical simulations. We consider the planar injection of a less viscous fluid into a two-dimensional fractured porous medium that is saturated with a more viscous fluid. This problem contains two sets of fundamentally different preferential flow regimes; the first is caused by the viscous fingering, and the second is due to the permeability contrasts between the fractures and the rock matrix. We study the transition from the regime where the flow is dominated by the viscous instabilities, to the regime where the heterogeneity induced by the fractures define the flow paths. Our findings reveal that even minor permeability differences between the rock matrix and fractures significantly influence the behavior of viscous fingering. The interplay between the viscosity contrast and permeability contrast leads to the preferential channeling of the less viscous fluid through the fractures. Consequently, this channeling process stabilizes the displacement front within the rock matrix, ultimately suppressing the occurrence of viscous fingering, particularly for higher permeability contrasts. We explore three fracture geometries: two structured and one random configuration and identify a complex interaction between these geometries and the development of unstable flow. While we find that the most important factor determining the effect of the fracture network is the ratio of fluid volume flowing through the fractures and the rock matrix, the exact point for the cross-over regime is dependent on the geometry of the fracture network.

Catalytic Cycle of the Bifunctional Enzyme Phosphoribosyl-ATP Pyrophosphohydrolase/Phosphoribosyl-AMP Cyclohydrolase.

The bifunctional enzyme phosphoribosyl-ATP pyrophosphohydrolase/phosphoribosyl-AMP cyclohydrolase (HisIE) catalyzes the second and third steps of histidine biosynthesis: pyrophosphohydrolysis of N1-(5-phospho-β-D-ribosyl)-ATP (PRATP) to N1-(5-phospho-β-D-ribosyl)-AMP (PRAMP) and pyrophosphate in the C-terminal HisE-like domain, and cyclohydrolysis of PRAMP to N-(5'-phospho-D-ribosylformimino)-5-amino-1-(5″-phospho-D-ribosyl)-4-imidazolecarboxamide (ProFAR) in the N-terminal HisI-like domain. Here we use UV-VIS spectroscopy and LC-MS to show Acinetobacter baumannii putative HisIE produces ProFAR from PRATP. Employing an assay to detect pyrophosphate and another to detect ProFAR, we established the pyrophosphohydrolase reaction rate is higher than the overall reaction rate. We produced a truncated version of the enzyme-containing only the C-terminal (HisE) domain. This truncated HisIE was catalytically active, which allowed the synthesis of PRAMP, the substrate for the cyclohydrolysis reaction. PRAMP was kinetically competent for HisIE-catalyzed ProFAR production, demonstrating PRAMP can bind the HisI-like domain from bulk water, and suggesting that the cyclohydrolase reaction is rate-limiting for the overall bifunctional enzyme. The overall kcat increased with increasing pH, while the solvent deuterium kinetic isotope effect decreased at more basic pH but was still large at pH 7.5. The lack of solvent viscosity effects on kcat and kcat/KM ruled out diffusional steps limiting the rates of substrate binding and product release. Rapid kinetics with excess PRATP demonstrated a lag time followed by a burst in ProFAR formation. These observations are consistent with a rate-limiting unimolecular step involving a proton transfer following adenine ring opening. We synthesized N1-(5-phospho-β-D-ribosyl)-ADP (PRADP), which could not be processed by HisIE. PRADP inhibited HisIE-catalyzed ProFAR formation from PRATP but not from PRAMP, suggesting that it binds to the phosphohydrolase active site while still permitting unobstructed access of PRAMP to the cyclohydrolase active site. The kinetics data are incompatible with a build-up of PRAMP in bulk solvent, indicating HisIE catalysis involves preferential channeling of PRAMP, albeit not via a protein tunnel.

Yeasts of the Blastobotrys genus are promising platform for lipid-based fuels and oleochemicals production.

Strains of the yeast genus Blastobotrys (subphylum Saccharomycotina) represent a valuable biotechnological resource for basic biochemistry research, single-cell protein, and heterologous protein production processes. Species of this genus are dimorphic, non-pathogenic, thermotolerant, and can assimilate a variety of hydrophilic and hydrophobic substrates. These can constitute a single-cell oil platform in an emerging bio-based economy as oleaginous traits have been discovered recently. However, the regulatory network of lipogenesis in these yeasts is poorly understood. To keep pace with the growing market demands for lipid-derived products, it is critical to understand the lipid biosynthesis in these unconventional yeasts to pinpoint what governs the preferential channelling of carbon flux into lipids instead of the competing pathways. This review summarizes information relevant to the regulation of lipid metabolic pathways and prospects of metabolic engineering in Blastobotrys yeasts for their application in food, feed, and beyond, particularly for fatty acid-based fuels and oleochemicals. KEY POINTS: • The production of biolipids by heterotrophic yeasts is reviewed. • Summary of information concerning lipid metabolism regulationis highlighted. • Special focus on the importance of diacylglycerol acyltransferases encoding genes in improving lipid productionismade.

Heterogeneity-controlled uncertain optimization of pump-and-treat systems explained through geological entropy

Pump-and-treat (P&T) is a widely-adopted solution for the containment of solute plumes in contaminated aquifers. A cost-effective design of P&T systems requires optimizing (minimizing) the overall pumping rates (Q). This optimization is a stochastic process, as Q is a random variable linked to the randomness of the aquifer hydraulic conductivity (K). Previously presented stochastic approaches to minimize Q adopted two-dimensional (2D) Gaussian random spatial fields (r.s.f.) of log-transformed K. Recent studies based on geological entropy have demonstrated the limited ability of Gaussian r.s.f. to reproduce extreme K patterns, which mostly control transport in heterogeneous aquifers, when compared to non-Gaussian r.s.f. Moreover, 2D models generate different flow and transport connectivity than three-dimensional (3D) models. On these premises, this work aimed at extending previous works on P&T optimization in heterogeneous aquifers through Monte-Carlo groundwater simulations of 2D and 3D Gaussian and non-Gaussian r.s.f. The results indicated that the mean ( $$ \bar{Q}_{n} $$ ) and variance ( $$ \sigma_{Qn}^{2} $$ ) of the optimal Q distribution depend strictly on the chosen model dimensionality and r.s.f. generator. In particular, 2D models and models embedding indicator-based (i.e. non-Gaussian) r.s.f. tended to generate higher $$ \bar{Q}_{n} $$ and $$ \sigma_{Qn}^{2} $$ than 3D models with increasing number of model layers (KL) and Gaussian models. This behavior can be explained considering the spatial ordering of K clusters in the simulated aquifers, which is measured through metrics derived from the concept of geological entropy. It was found that 2D models and models embedding non-Gaussian r.s.f. displayed more spatially-persistent ordered K structures than 3D models and Gaussian models, resulting in higher $$ \bar{Q}_{n} $$ and $$ \sigma_{Qn}^{2} $$ . This is attributed to the relative amount of heterogeneity sampled by the solute source and the increased likelihood of more ordered K clusters to generate preferential flow and solute transport channeling than more disordered and chaotic systems, which enhance solute mixing. Combining P&T with physical barriers (i.e. cut-off walls) was helpful to reduce both $$ \bar{Q}_{n} $$ and $$ \sigma_{Qn}^{2} $$ in all tested scenarios, corroborating previous findings. However, the relative efficacy of a specific physical barrier geometry to reduce $$ \bar{Q}_{n} $$ and $$ \sigma_{Qn}^{2} $$ also depends on the chosen model dimensionality and r.s.f. generator.

Numerical Simulation of Unstable Preferential Flow during Water Infiltration into Heterogeneous Dry Soil

Water infiltration and unsaturated flow through heterogeneous soil control the distribution of soil moisture in the vadose zone and the dynamics of groundwater recharge, providing the link between climate, biogeochemical soil processes and vegetation dynamics. Infiltration into dry soil is hydrodynamically unstable, leading to preferential flow through narrow wet regions (fingers). In this paper we use numerical simulation to study the interplay between fingering instabilities and soil heterogeneity during water infiltration. We consider soil with heterogeneous intrinsic permeability. Permeabilities are random, with point Gaussian statistics, and vary smoothly in space due to spatial correlation. The key research question is whether the presence of moderate or strong heterogeneity overwhelms the fingering instability, recovering the simple stable displacement patterns predicted by most simplified model of infiltration currently used in hydrological models from the Darcy to the basin scales. We perform detailed simulations of constant-rate infiltration into soils with isotropic and anisotropic intrinsic permeability fields. Our results demonstrate that soil heterogeneity does not suppress fingering instabilities, but it rather enhances its effect of preferential flow and channeling. Fingering patterns strongly depend on soil structure, in particular the correlation length and anisotropy of the permeability field. While the finger size and flow dynamics are only slightly controlled by correlation length in isotropic fields, layering leads to significant finger meandering and bulging, changing arrival times and wetting efficiencies. Fingering and soil heterogeneity need to be considered when upscaling the constitutive relationships of multiphase flow in porous media (relative permeability and water retention curve) from the finger to field and basin scales. While relative permeabilities remain unchanged upon upscaling for stable displacements, the inefficient wetting due to fingering leads to relative permeabilities at the field scale that are significantly different from those at the Darcy scale. These effective relative permeability functions also depend, although less strongly, on heterogeneity and soil structure.

Pore‐Scale Dissolution by CO2 Saturated Brine in a Multimineral Carbonate at Reservoir Conditions: Impact of Physical and Chemical Heterogeneity

AbstractWe study the impact of physical and chemical heterogeneity on reaction rates in multimineral porous media. We selected two pairs of carbonate samples of different physical heterogeneity in accordance with their initial computed velocity distributions and then injected CO2 saturated brine at reservoir conditions at two flow rates. We periodically imaged the samples using X‐ray microtomography. The mineralogical composition was similar (a ratio of dolomite to calcite of 8:1), but the intrinsic reaction rates and mineral spatial distribution were profoundly different. Visualizations of velocity fields and reacted mineral distributions revealed that a dominant flow channel formed in all cases. The more physically homogeneous samples had a narrower velocity distribution and more preexisting fast channels, which promoted dominant channel formation in their proximity. In contrast, the heterogeneous samples exhibit a broader distribution of velocities and fewer fast channels, which accentuated nonuniform calcite distribution and favored calcite dissolution away from the initially fast pathways. We quantify the impact of physical and chemical heterogeneity by computing the proximity of reacted minerals to the fast flow pathways. The average reaction rates were an order of magnitude lower than the intrinsic ones due to mass transfer limitations. The effective reaction rate of calcite decreased by an order of magnitude, in both fast channels and slow regions. After channel formation calcite was shielded by dolomite whose effective rate in slow regions could even increase. Overall, the preferential channeling effect, as opposed to uniform dissolution, was promoted by a higher degree of physical and/or chemical heterogeneity.

Elevation of blood lipids in hepatocyte-specific fatty acid transport 4-deficient mice fed with high glucose diets

Fatty acid transport protein4 (FATP4) is upregulated in acquired and central obesity and its polymorphisms are associated with blood lipids and insulin resistance. Patients with FATP4 mutations and mice with global FATP4 deletion exhibit skin abnormalities characterized as ischthyosis prematurity syndrome (IPS). Cumulating data have shown that an absence of FATP4 increases the levels of cellular triglycerides (TG). However, FATP4 role and consequent lipid and TG metabolism in the hepatocyte is still elusive. Here, hepatocyte-specific FATP4 deficient (Fatp4L−/−) mice were generated. When fed with chow, these mutant mice displayed no phenotypes regarding blood lipids. However when fed low-fat/high-sugar (HS) or high-fat/high-sugar (HFS) for 12 weeks, Fatp4L−/− mice showed a significant increase of plasma TG, free fatty acids and glycerol when compared with diet-fed control mice. Interestingly, Fatp4L−/− mice under HS diet had lower body and liver weights and they were not protected from HFS-induced body weight gain and hepatic steatosis. Male mutant mice were more sensitive to HFS diet than female mutant mice. Glucose intolerance was observed only in female Fatp4L−/− mice fed with HS diet. Lipidomics analyses revealed that hepatic phospholipids were not disturbed in mutant mice under both diets. Thus, hepatic FATP4 deletion rendered an increase of blood lipids including glycerol indicating a preferential fatty-acid channeling to TG pools that are specifically available for lipolysis. Our results imply a possible risk of hyperlipidemia as a result of abnormal metabolism in liver in IPS patients with FATP4 mutations who consume high-sugar diets.

Numerical simulation of the impact of geological heterogeneity on performance and safety of THAI heavy oil production process

The Toe-to-Heel Air Injection (THAI) in-situ combustion process is an efficient way to extract heavy oil and bitumen. However, such reservoirs are often geologically heterogeneous. This work studied the impact of a range of different geological heterogeneities, often found in bitumen deposits, on the performance and safety of THAI. These heterogeneities included random heterogeneity, layered reservoirs, shaly reservoirs, and semi-permeable cap-rocks. A further aim was to also develop potential remedial measures, such as selective well placement. It was found that the degree of symmetry assumed for the reservoir model had a substantial impact on the predicted level of oil production. This is of particular relevance to otherwise apparently symmetrical well placement designs such as staggered line drive. While the presence of impermeable zones resulted in the decrease in the overall oxygen utilisation for shaly reservoirs, compared to simply low permeability reservoirs, there was no evidence of oxygen breakthrough due to preferential channelling into the production well. In layered reservoirs, the development of a rich oil bank during THAI operation depended upon the distribution of permeability around the horizontal producer (HP), and did not occur when there was high permeability just above the HP. It has been shown that the proper representation of the cap-rock in reservoir models for the simulation of THAI is essential in order to accurately mimic the full pattern of heat distribution into the oil zone of the reservoir, and, thence, fuel lay-down. While THAI can operate stably with a permeable cap-rock, vertical permeabilities above ∼1–3 mD led to significant loss of combustion gases from the reservoir.

A study of uranium-ore agglomeration parameters and their implications during heap leaching

Agglomeration is commonly used for processing clay-rich ores in order to prevent undesirable effects, especially the risks of clogging or preferential channelling within the heap leaching piles. Several parameters such as agglomeration time, agglomerates water content or agglomerates binder content were identified as having a strong influence on the agglomeration process. In the present study, the impact of water content, sulfuric acid and addition of a polyacrylamide binder on the porous structure and mechanical strength of uranium-ore agglomerates were investigated, before as well as after 10 days of leaching. A multi-analytical approach, combining SEM, MIP, ICP-OES, X-ray tomography analyses and oedometer tests was used for this purpose. Increasing sulfuric acid concentration at agglomeration was found to enhance uranium extraction extent but reducing agglomerate strength during leaching. In addition, the increase of L/S ratio caused a decrease of agglomerate porosity due to the formation of higher amount of primary aluminous silicate matrix. Finally, the use of a polyacrylamide binder improved leaching resistance of agglomerates, allowing an increase of both agglomerate porosity and uranium extraction rates between 24 and 240 h of leaching while keeping a better compressive strength.

Preferential flow behaviour in unsaturated packed beds and heaps: Incorporating into a CFD model

Heap leach stockpiles inevitably contain local voidage heterogeneities due to non-uniform particle size distributions of the ore and other factors that lead to preferential flow paths and solution channelling. The stockpile can also encounter diverse flow conditions due to a number of factors, including storm events, infiltration into dry ore material, cyclic drain down, compaction, migration of fines, all contributing to large variations in local ore permeability and the creation of preferential flow pathways. Non-uniform and adverse flow behaviour within the heap reduces the leaching efficiency which can lead to lower metal recoveries. Therefore, capturing the local flow variations that affect the transport of leach solution within the heap is critical to accurately predicting the leaching kinetics. Experimental data shows how channelling develops due to local heterogeneities that cannot be eliminated by packing alone. Thus, effective modelling of heap leach stockpiles should account for these channelling affects. This paper utilises a robust computational fluid dynamics (CFD) framework that incorporates techniques to account for local preferential flow paths in the heap leach system. The results are compared against liquid flow behaviour in a pseudo two-dimensional column of narrowly sized particles and a more realistic particle size distribution. The methods are then applied to a hypothetical leach to assess the impact of accounting for the flow variability in the heap.

A multi-analytical approach to the study of uranium-ore agglomerate structure and porosity during heap leaching

Agglomeration is commonly used for processing clay-rich ores in order to prevent undesirable effects, especially the risks of plugging or preferential channelling within the heap leaching piles. As the mechanical and chemical stability of the agglomerates is of crucial importance for the behaviour of the pile, a detailed characterization of the structure and porosity of agglomerates seems necessary. In the present study, uranium-bearing clay rich ore was agglomerated and agglomerates were studied before as well as after 10days of leaching. A multi-analytical approach (by X-ray tomography, SEM, XRD and MIP analyses) was used for the study of the mineralogy, porosity distribution and textures of the agglomerates. X-ray tomography and SEM images showed that agglomerates were the result of coalescence and layering of micro-agglomerates where each one in turn was composed of a nucleus embedded within phyllosilicates and an aluminous silicate matrix. MIP analyses highlighted that unleached agglomerates had a low connected porosity (between 3 and 7%). During the 24 first hours of process, the connected porosity increased by a factor 2 because of the leaching of the aluminous silicate matrix and about 60% of the uranium was recovered. During the next 9days, the formation of a secondary aluminous silicate matrix derived from the dissolution of illites resulted in the plugging of mesopores and therefore led to a decrease of porosity and uranium extraction extent.

Implications of Material Reponse for Battery Safety

There has been a significant amount of effort to make lithium ion batteries safer, resulting in improvements across the board from safer cathode chemistries, alternate candidates for anode materials, electrolytes with wider operating windows, separators with better mechanical and thermal properties, to binders that are less reactive under abuse conditions. At the cell level, safety improvements include tabs that limit electrical conductivity at high temperatures, alternate pathways to dissipate heat and/or abuse reaction currents and preferential channeling of vent gases. Test methodologies have improved a great deal over the last few years: we are able to selectively investigate the effect of SEI additives on abuse response of the cell in situ and obtain tomographic images during thermal runaway. All these capabilities have enabled our understanding of the interactions among the different cell components during cell abuse scenario considerably. As reported previously, improvements to individual components are not always adequate to address cell safety. Given the extent of advancements in characterization techniques and material properties, the next logical step is to understand the influence of material-scale-improvements on the cell level response during short-circuit. However, this step is not straightforward. There are several chemical/ electrochemical reactions involving multiple components that take place simultaneously. Material properties such as thermal diffusivity and electrical conductivity change during both nominal operation as well as during the abuse reactions. In this study, we present a rigorous characterization of the reactivity, as well as material properties with aging of the cell. We then proceed to use these properties to better explain the temperature distribution within the cell during short-circuit. Implications of changes in material properties for propagation of thermal runaway at the cell level are described based on experimental observations. The results are used in mathematical models to assess effectiveness of different design options in improving cell-level safety.

Roles of Acyl-CoA:Diacylglycerol Acyltransferases 1 and 2 in Triacylglycerol Synthesis and Secretion in Primary Hepatocytes.

Very low-density lipoprotein assembly and secretion are regulated by the availability of triacylglycerol. Although compelling evidence indicates that the majority of triacylglycerol in very low-density lipoprotein is derived from re-esterification of lipolytic products released by endoplasmic reticulum-associated lipases, little is known about roles of acyl-CoA:diacylglycerol acyltransferases (DGATs) in this process. We aimed to investigate the contribution of DGAT1 and DGAT2 in lipid metabolism and lipoprotein secretion in primary mouse and human hepatocytes. We used highly selective small-molecule inhibitors of DGAT1 and DGAT2, and we tracked storage and secretion of lipids synthesized de novo from [(3)H]acetic acid and from exogenously supplied [(3)H]oleic acid. Inactivation of individual DGAT activity did not affect incorporation of either radiolabeled precursor into intracellular triacylglycerol, whereas combined inactivation of both DGATs severely attenuated triacylglycerol synthesis. However, inhibition of DGAT2 augmented fatty acid oxidation, whereas inhibition of DGAT1 increased triacylglycerol secretion, suggesting preferential channeling of separate DGAT-derived triacylglycerol pools to distinct metabolic pathways. Inactivation of DGAT2 impaired cytosolic lipid droplet expansion, whereas DGAT1 inactivation promoted large lipid droplet formation. Moreover, inactivation of DGAT2 attenuated expression of lipogenic genes. Finally, triacylglycerol secretion was significantly reduced on DGAT2 inhibition without altering extracellular apolipoprotein B levels. Our data suggest that DGAT1 and DGAT2 can compensate for each other to synthesize triacylglycerol, but triacylglycerol synthesized by DGAT1 is preferentially channeled to oxidation, whereas DGAT2 synthesizes triacylglycerol destined for very low-density lipoprotein assembly.

The Early Permian Tarim Large Igneous Province: Main characteristics and a plume incubation model

The Early Permian magmatism in Tarim, NW China comprises diamondiferous kimberlites, lamprophyres, flood basalts, Fe–Ti oxide ore-bearing layered mafic–ultramafic intrusions, bi-modal dyke swarms, alkaline igneous complexes (including syenites and A-type granites), rhyolites and pyroclastic rocks. The extent of this intraplate magmatism exceeds 250,000 km 2 , making it comparable to Large Igneous Provinces (LIPs). Screening of available radiometric ages reveals three main magmatic episodes in the Tarim LIP, with the first being marked by ~ 300 Ma small-volume kimberlites, followed by two phases of bimodal magmatism at ~ 290 Ma and at ~ 280 Ma, respectively. This relatively long time interval of the Early Permian magmatism is consistent with a low eruption rate of the Tarim LIP and is supported by the intercalation of volcanic rocks with sediments in outcrops and drill holes. Although the spatial distribution of each magmatic episode in the Tarim LIP is far from assessed, it seems that the ~ 290 Ma flood basalts are widespread across the province, whereas ~ 300 Ma kimberlites and ~ 280 Ma ultramafic–mafic–felsic intrusions and dyke swarms only occur in the Bachu Uplift and around the margins of the Tarim craton. We propose that the ~ 300 Ma kimberlites were derived from deep part of the metasomatized sub-continental lithospheric mantle (SCLM), while the ~ 290 Ma flood basalts were likely formed as a result of mixing of plume-derived melts with SCLM-derived melts (e.g., lamproitic melt) as they rose through the SCLM. In contrast, the ~ 280 Ma magmas were most likely derived from the convecting mantle. A plume incubation model is proposed to account for the temporo-spatial distribution of the Tarim LIP, in which different styles of plume–lithosphere interaction are recognized. In the first two episodes, the mantle plume incubating the base of the craton provides the heat that triggered melting of the enriched components in the SCLM. In contrast, adiabatic decompression melting within the plume produced the ~ 280 Ma magmatic phase. Thermal modeling suggests that lithospheric thinning by thermal erosion might have been associated with the upwelling mantle plume, with the greatest thinning occurring in the Bachu area. Thinned spots and weak zones at the margins of cratons and mobile belts caused preferential channeling of plume flow and subsequent decompression melting. This explains the localized distribution of ~ 280 Ma magmas in the Tarim LIP. • Spatial distribution of igneous rocks in the Tarim large igneous province • Three magmatic episodes possess distinct rock association and geochemistry. • Lithospheric thinning in association with thermal erosion by upwelling mantle • Plume–lithosphere interaction in the formation of the Tarim LIP

Periodic Feedwater Reversal and Air Sparging As Antifouling Strategies in Reverse Electrodialysis

Renewable energy can be generated using natural streams of seawater and river water in reverse electrodialysis (RED). The potential for electricity production of this technology is huge, but fouling of the membranes and the membrane stack reduces the potential for large scale applications. This research shows that, without any specific antifouling strategies, the power density decreases in the first 4 h of operation to 40% of the originally obtained power density. It slowly decreases further in the remaining 67 days of operation. Using antifouling strategies, a significantly higher power density can be maintained. Periodically switching the feedwaters (i.e., changing seawater for river water and vice versa) generates the highest power density in the first hours of operation, probably due to a removal of multivalent ions and organic foulants from the membrane when the electrical current reverses. In the long term, colloidal fouling is observed in the stack without treatment and the stack with periodic feedwater switching, and preferential channeling is observed in the latter. This decreases the power density further. This decrease in power density is partly reversible. Only a stack with periodic air sparging has a minimum of colloidal fouling, resulting in a higher power density in the long term. A combination of the discussed antifouling strategies, together with the use of monovalent selective membranes, is recommended to maintain a high power density in RED in short-term and long-term operations.

Early detection of preferential channeling in reverse electrodialysis

Membrane applications often experience fouling, which prevent uniform flow distribution through the feed water compartments, i.e. preferential channeling may occur. This research shows the effect of preferential channeling on energy generation from mixing salt water and fresh water using reverse electrodialysis (RED). The experimentally obtained power density, electrical resistance and pressure drop are evaluated for artificially controlled preferential channeling. The obtained power density decreases significantly when part of the feed water compartment is inaccessible for flow; a blockage of only 10% of the feed water compartments decreases the net power density by approximately 20%. When 80% of the feed water channels is inaccessible, the net power densities are only marginally positive. This decrease in power density is due to an increase in non-ohmic resistance, which is related to the concentration changes in the feed water compartments when ions are transported from the seawater to the river water side. Chronopotentiometric measurements show that the typical response time to establish a non-ohmic overpotential is an even more sensitive and easily scalable parameter to detect preferential channeling (and the presence of possible fouling) in an early stage. In practical applications, this response time can thus be used as an indicator for preferential channeling and serve to decide on and selectively apply cleaning in RED and other applications.

Influence of air flow rate and backwashing on the hydraulic behaviour of a submerged filter

The aim of this study was to evaluate backwashing effects on the apparent porosity of the filter media and on the hydraulic behaviour of a pilot scale submerged filter, prior to biofilm colonization, under different hydraulic retention times, and different air flow rates. Tracer curves were analysed with two mathematical models for ideal and non-ideal flow (axial dispersion and Wolf and Resnick models). The filter media was lava stones sieved to 4.5 mm. Backwashing causes attrition of media particles, decreasing the void volume of the filter media and, consequently, the tracer flow is more uniform. The eroded media presented lower dead volumes (79% for the filter with aeration and 8% for the filter without aeration) compared with the new media (83% for the filter with aeration and 22% for the filter without aeration). The flow patterns of eroded and new media were different because the more regular shape of the particles decreases the void volume of the filter media. The dead volume is attributed, in the case of the filter with aeration, to the turbulence caused by the air bubbles that generate preferential channelling of the bulk liquid along the filter media, creating large zones of stagnant liquid and, for the filter without aeration, to the channels formed due to the irregular shaped media.

Residence time distribution determination in a submerged filter with and without aeration using a tracer

AbstractBACKGROUNDCommercial filter media are commonly regular in shape and the flow patterns can be predicted with simple tools. Using lava stones as filter media, this study analyzes the influence of particle size and aeration on the hydraulic behaviour of a submerged filter prior to biofilm colonization.RESULTSThe filter was packed, separately, with two different sizes of lava stones (4.7 and 9.5 mm) and operated under different hydraulic retention times to establish the causes of deviations from ideal models. Tracer curves were analyzed using two mathematical models (Axial Dispersion and Wolf and Resnick models). The curves show that the tracer appeared in the effluent in a shorter time than expected indicating an effective volume reduction. It was estimated that the dead volume of the filter with aeration was 83% and 22% for the filter without aeration.CONCLUSIONThe dead volume is caused, in the case of the filter with aeration, by the combination of two phenomena: the liquid volume displaced by the air bubbles and the turbulence caused by aeration generating preferential channelling along the filter media and creating large zones of stagnant liquid. In the case of the filter without aeration the volume reduction is caused by channels developed by the media irregular shape. © 2013 Society of Chemical Industry

Geochemical evidence of mantle reservoir evolution during progressive rifting along the western Afar margin

The Afar triple junction, where the Red Sea, Gulf of Aden and African Rift System extension zones converge, is a pivotal domain for the study of continental-to-oceanic rift evolution. The western margin of Afar forms the southernmost sector of the western margin of the Red Sea rift where that margin enters the Ethiopian flood basalt province. Tectonism and volcanism at the triple junction had commenced by ∼31Ma with crustal fissuring, diking and voluminous eruption of the Ethiopian-Yemen flood basalt pile. The dikes which fed the Oligocene-Quaternary lava sequence covering the western Afar rift margin provide an opportunity to probe the geochemical reservoirs associated with the evolution of a still active continental margin. 40Ar/39Ar geochronology reveals that the western Afar margin dikes span the entire history of rift evolution from the initial Oligocene flood basalt event to the development of focused zones of intrusion in rift marginal basins. Major element, trace element and isotopic (Sr-Nd-Pb-Hf) data demonstrate temporal geochemical heterogeneities resulting from variable contributions from the Afar plume, depleted asthenospheric mantle, and African lithosphere. The various dikes erupted between 31Ma and 22Ma all share isotopic signatures attesting to a contribution from the Afar plume, indicating this initial period in the evolution of the Afar margin was one of magma-assisted weakening of the lithosphere. From 22Ma to 12Ma, however, diffuse diking during continued evolution of the rift margin facilitated ascent of magmas in which depleted mantle and lithospheric sources predominated, though contributions from the Afar plume persisted. After 10Ma, magmatic intrusion migrated eastwards towards the Afar rift floor, with an increasing fraction of the magmas derived from depleted mantle with less of a lithospheric signature. The dikes of the western Afar margin reveal that magma generation processes during the evolution of this continental rift margin are increasingly dominated by shallow decompressional melting of the ambient asthenosphere, the composition of which may in part be controlled by preferential channeling of plume material along the developing neo-oceanic axes of extension.

On Preferential Flow, Channeling and Connectivity in Heterogeneous Porous Formations

This paper analyzes the emergence of channeling and preferential flow in heterogeneous porous media. Connectivity is studied through the statistical characterization of the length L of connected, high velocity patterns in both two-dimensional and three-dimensional media. A simple, physically based, fully analytic expression for the probability of L has been derived. It is found that the length L of connected, high velocity channels is flow-related and can be much larger than the conductivity integral scale I. Heterogeneity has a considerable impact on emergence of channeling patterns; connectivity is considerably enhanced in three-dimensional structures as compared to two-dimensional ones. The strong dependence on space dimensionality is a warning against the use of two-dimensional numerical models for assessing connectivity and preferential flow in heterogeneous media. The probability p(L) is employed in order to determine the early arrivals of the breakthrough curve at a given control plane; the simple model can be used for a preliminary assessment of preferential flow. Comparison with numerical simulations confirms that the main connectivity features were adequately captured by the model.