Scanning Curves Research Articles

Modeling water vapor adsorption/desorption cycles

This modeling work deals with the adsorption of water vapor on different porous materials where it undergoes capillary condensation and its adsorption/desorption isotherms exhibit hysteresis. The focus is on the description of the so called scanning curves, i.e. the adsorption/desorption isotherms observed when such an adsorbent is repeatedly loaded and unloaded in a range of conditions where hysteresis is observed, and on the simulation of fixed bed adsorption/desorption cycles. We use an approach originally developed by Štěpánek et al. (Chem Eng Sci 55(2):431–440, 2000), and expand it so as to include more general isotherms (not only the Dubinin–Radushkevich and Dubinin–Astakhov model, but also the Guggenheim–Anderson–de Boer model and the Do and Do model) and to allow for less than infinitely fast heat transfer, so as to consider non-isothermal situations. From a modeling point of view the results are satisfactory and highlight the need for better experimental data on water vapor adsorption, which need to be measured in enhanced experimental set-ups, capable to tightly control the relative humidity of the gas phase.

Effects of Carbon Coating and Pore Corrugation on Capillary Condensation of Nitrogen in SBA-15 Mesoporous Silica

To examine the origin of an ink-bottle-like structure in SBA-15 formed by carbon coating and the effects of pore corrugation on capillary condensation and evaporation of a vapor in the cylindrical pores, we measured the adsorption isotherms of nitrogen at 77 K on 10 kinds of SBA-15 samples before and after a carbon coating process by the exposure to acetylene at 1073 K, as well as desorption scanning curves and subloops on the untreated samples. These SBA-15 samples were synthesized under the different conditions of initial SiO2/P123 ratio and hydrothermal treatment. SBA-15 with relatively large microporosity tends to form easily constrictions inside the main channels by the carbon coating. This strongly suggests that the rough pore walls of SBA-15 may induce the incomplete wetting of carbon layers on the pore walls to form the constrictions inside the cylindrical pores. A comparison of two subloops implies that the pores of SBA-15 synthesized with a SiO2/P123 ratio of 75 consist of an assembly of connecting domains of different diameters; that is, the pores are highly corrugated. For SBA-15 synthesized with a SiO2/P123 ratio of 60, the amplitude of the pore corrugation is significantly decreased by the prolonged hydrothermal treatment at 373 K. On the other hand, for SBA-15 synthesized with a SiO2/P123 ratio of 45, the amplitude of the corrugation is negligibly small, although the cylindrical pores are interconnected through narrow necks with each other. It is found that the smaller the amplitude of the pore corrugation, the smaller the width of the hysteresis loop.

Variations in Bounding and Scanning Relative Permeability Curves With Different Carbonate Rock Types

Summary Relative permeability curves generally exhibit hysteresis between different saturation cycles. This hysteresis is mainly caused by wettability changes and fluid trapping. Different rock types may experience different hysteresis trends because of variations in pore geometry. Relative permeability curves may also be a function of the saturation height in the reservoir. A detailed laboratory study was performed to investigate relative permeability behavior for a major carbonate hydrocarbon reservoir in the Middle East. Representative core samples covering five reservoir rock types (RRTs) were identified on the basis of whole core and plug X-ray computed tomography (CT), nuclear magnetic resonance (NMR) T2, mercury injection capillary pressure (MICP), porosity, permeability, and thin-section analyses. Primary-drainage (PD) and imbibition water/oil relative permeability (bounding) curves were measured on all the five rock types by the steady-state (SS) technique by use of live fluids at full reservoir conditions with in-situ saturation monitoring (ISSM). Imbibition relative permeability experiments were also conducted on the main RRT samples to assess the relative permeability (scanning) curves in the transition zone (TZ) by varying connate-water saturations. Hysteresis effects were observed between PD and imbibition cycles, and appeared to be influenced by the sample rock type involved (i.e., wettability and pore geometry). Variations in relative permeability within similar and different rock types were described and understood from local heterogeneities present in each individual sample. This was possible from dual-energy (DE) CT scanning and high-resolution imaging. Different imbibition trends from both oil and water phases were detected from the scanning curves that were explained by different pore-level fluid-flow scenarios. Relative permeability scanning curves to both oil and water phases increased with higher connate-water saturation. Relative permeability to oil was explained on the basis of the occupancy of the oil phase at varying connate-water saturations. The change in the water relative permeability trend was addressed on the basis of the connectivity of water at the varying connate-water saturations. These results and interpretations introduced an improved understanding of the hysteresis phenomena and fluid-flow behavior in the TZ of a Cretaceous carbonate reservoir that can assist in the overall reservoir modeling and well planning.

Trapping and hysteresis in two‐phase flow in porous media: A pore‐network study

Several models for two‐phase flow in porous media identify trapping and connectivity of fluids as an important contribution to macroscale hysteresis. This is especially true for hysteresis in relative permeabilities. The trapping models propose trajectories from the initial saturation to the end saturation in various ways and are often based on experiments or pore‐network model results for the endpoints. However, experimental data or pore‐scale model results are often not available for the trajectories, that is, the fate of the connectivity of the fluids while saturation changes. Here, using a quasi static pore‐network model, supported by a set of pore‐scale laboratory experiments, we study how the topology of the fluids changes during drainage and imbibition including first, main and scanning curves. We find a strong hysteretic behavior in the relationship between disconnected nonwetting fluid saturation and the wetting fluid saturation in a water‐wet medium. The coalescence of the invading nonwetting phase with the existing disconnected nonwetting phase depends critically on the presence (or lack thereof) of connected nonwetting phase at the beginning of the drainage process as well as on the pore geometry. This dependence involves a mechanism we refer to as “reversible corner filling.” This mechanism can also be seen in laboratory experiments in volcanic tuff. The impact of these pore‐network model results on existing macroscopic models is discussed.

Measurement and modeling of moisture content above an oscillating water table: implications for beach surface moisture dynamics

ABSTRACTThis study examined the influence of tidally‐induced oscillations of the beach water table in regulating beach surface moisture dynamics. A series of laboratory experiments were conducted to assess the influence of hysteresis and transient flow effects on surface moisture variability. The experimental apparatus utilized a column of well‐sorted fine sand partially immersed in a reservoir of water. The water level in the reservoir was raised and lowered via a diaphragm‐metering pump to simulate tidally induced fluctuations of the water table, and the moisture content profile within the column was monitored using an array of Delta‐T probes. Moisture contents at specific elevations within the column were utilized as proxies to represent various ‘surface’ elevations (relative to the high water table). Results indicate that surface moisture content behaves in a distinctly hysteretic manner. Examination of water flow scanning curves illustrated that for all surface elevations considered, higher moisture contents for a given pressure head occurred during the drying cycle than during the wetting cycle. This observation is particularly evident with shallow surface elevations (i.e. water table close to the surface) where the Haines Jump phenomenon was found to have a significant influence on moisture content dynamics. Additionally, an assessment of the accuracy of hysteretic and non‐hysteretic models to predict the measured moisture contents demonstrated that hysteretic simulations consistently provide a better representation of the observed moisture contents than non‐hysteretic simulations. A time lag was found between the respective maxima and minima in water table elevation surface moisture content. At the near surface water table positions the time lag ranged between 30 and 100 minutes, and it increased to 240 minutes (four hours) with the high water table at 60 cm below the surface. Copyright © 2013 John Wiley & Sons, Ltd.

Probing hysteresis during sorption of cyclohexane within mesoporous silica using NMR cryoporometry and relaxometry

The conventional data analysis methods for obtaining a pore size distribution (PSD) from gas sorption data make several critical assumptions that impact significantly on the accuracy of the PSD thereby obtained. In particular, assumptions must be made concerning the nature of the pore-filling or emptying process in adsorption, or desorption, respectively. The possibility of pore–pore interactions is also generally neglected. In this work, NMR cryoporometry and relaxometry have been used to study the adsorption and desorption of cyclohexane within a mesoporous, sol–gel silica catalyst support pellet with the aim of assessing the impact of the aforementioned problems for gas sorption PSDs and developing solutions. The advanced melting effect makes cryoporometry a particularly sensitive probe of adsorbate ganglia spatial distribution. It has been demonstrated that utilising gas sorption scanning curves provided insufficient additional information to alleviate the aforementioned problems with interpreting gas sorption data. The NMR data has shown how the nature of the sorption hysteresis changed with amount adsorbed, due to detectable variations in the mechanisms of pore-filling and emptying along the isotherm. Hence, relating a particular condensation or evaporation pressure to a specific characteristic pore size is not as straightforward as assumed in typical pore size analysis software. However, the NMR techniques reveal the additional information required to improve pore size estimates from gas sorption for disordered solids.

Pyrolysis Products of Lignin at Different Temperature Detected by Py-GC-MS

Pyrolyzation of lignin to aromatic compounds with small molecule weight is one of the ways to efficient utilization of lignin resource. Basing on the data of thermogravimetric scanning curve (TG-DTG) and pyrolysis-gas chromatography -mass spectrometry (Py-GC-MS), the change in the distribution of pyrolysis products at different temperature was analyzed. The results showed that lignin could produce a large number of aromatic compounds having hydroxy or methoxy groups under the conditions of anaerobic and 250-450°C, but it was not benefit to obtain useful organic compounds when pyrolyzed at elevated temperatures.

InAs/GaSb type-II superlattice mid-wavelength infrared focal plane array detectors grown by molecular beam epitaxy

We report our recent works on the growth and fabrication of InAs/GaSb type II superlattice detectors by solid source molecular beam epitaxy. Lattice mismatch between the superlattice and the GaSb substrate reached 1.5×10−4. The full width at half maximum of the first order satellite peak from X-ray diffraction was 28″. Full simulation of the X-ray scanning curves showed the detailed composition of the InAsSb interface layers. Single element detectors and focal plane arrays were fabricated using wet chemical etching. The devices with 10ML InAs/10ML GaSb in each superlattice period showed a 50% cutoff wavelength of 5.4μm at 77K. The dark current density at −20mV bias was 3.8×10−7A/cm2 and the resistance–area product at zero bias 7.5×104Ωcm2.The quantum efficiency was measured to be 25%. The blackbody detectivity and the peak wavelength detectivity were 7.6×1011cmHz1/2/W and 3×1012cmHz1/2/W, respectively. The focal plane array was characterized with an integration time of 2.7ms and F/2.0 optics. The percentage of the dead pixels was only 1.8%, non-uniformity 5.41% and a noise equivalent differential temperature 33.4mK.

Relative permeability hysteresis and capillary trapping characteristics of supercritical CO2/brine systems: An experimental study at reservoir conditions

We present the results of an experimental study on the effects of hysteresis on capillary trapping and relative permeability of CO2/brine systems at reservoir conditions. We performed thirty unsteady- and steady-state drainage and imbibition full-recirculation flow experiments in three different sandstone rock samples, low- and high-permeability Berea and Nugget sandstones. The experiments were carried out at various flow rates with both supercritical CO2 (scCO2)/brine and gaseous CO2 (gCO2)/brine fluid systems. The unsteady-state experiments were carried out with a wide range of flow rates to establish a broad range of initial brine saturations (Swi). This allowed investigation of the sensitivity of residual trapped CO2 saturation (SCO2r) to changes in Swi. The values were successfully compared with those available in the literature. For a given Swi, the trapped scCO2 saturation was less than that of gCO2 in the same sample. This was attributed to brine being less wetting in the presence of scCO2 than in the presence of gCO2. Post-imbibition dissolution of trapped CO2 and formation of dissolution front was also investigated. During the steady-state experiments, scCO2 and brine were co-injected with monotonically increasing or decreasing fractional flows to perform drainage and imbibition processes. We carried out seven sets of steady-state flow tests with various trajectories generating a comprehensive group of relative permeability hysteresis curves. The scanning curves revealed distinct features with potentially important implications for storage of scCO2 in geological formations. For both series of experiments, the ratio of SCO2r to initial CO2 saturation (1- Swi) was found to be much higher for low initial CO2 saturations. The results indicate that very promising fractions (about 49 to 83%) of the initial CO2 saturation can be stored through capillary trapping.

Predicting Hysteresis of the Water Retention Curve from Basic Properties of Granular Soils

The water retention curve (WRC), which represents the relationship between volumetric water content (θ) and suction (ψ), is required to analyze the hydro-geotechnical response of unsaturated soils. The laboratory (or field) determination of the WRC can however be time consuming and difficult to conduct. A practical alternative, particularly useful at the preliminary stages of a project, is to estimate the WRC using a predictive model based on basic geotechnical properties that are easy to obtain. One common limitation of such predictive models is due to hysteresis effects, which are not taken into account by most of these models. The authors present in this paper an extended version of the Modified Kovacs (MK) predictive model that incorporates hysteresis of the WRC along different paths, including the main wetting and drying curves and the wetting and drying scanning curves for granular soils. The model formulation is presented, and predictions are compared to experimental data obtained on different granular soils. The results show a good agreement for the main and scanning curves.

Electrochemical Behavior of Carbon Steel in Neutral Chloride Solution Adding Amine-Carboxylate Based Corrosion Inhibitor

Studies have been carried out to investigate the inhibiting effect of organic corrosion inhibitor (OCI) on steel electrode in 1% NaCl solution using electrochemical techniques. The test results show that the commonly used inhibitors such as triethanolamine (TEA), dimethylethanolamine (AMA) and monofluorophosphate (MFP) as well as their complexes are efficient when used in the chloride solution, while an amine-carboxylate inhibitor (C12) prepared in lab bring an excellent inhibitive performance when added in the chloride solution. Based on the date of linear polarization resistance, the calculated lowest corrosion current of the steel electrode for 10d is about 0.32% of the initial current of the electrode in the chloride solution before adding C12 inhibitor.The date of potentiodynamic scanning curves also show that the inhibitor has self-repair ability even when the steel is localized corrosion attack. The Bode-phase diagram results suggest that the protective layer at the surface of steel becomes homogeneous and compact in the presence of chloride.

Synergism Between Alkylbenzene Sulfonate and Nonionic Surfactant for Producing Low Interfacial Tension at Oil/Water Interface

Synergism between alkylbenzene sulfonate and nonionic surfactant was studied by measuring interfacial tensions of surfactants and their mixtures against a homologous series of alkanes. Four di-n-alkylbenzene sulfonate homologues and a nonionic surfactant, lauryl alcohol polyoxyethylene(9) ether, were used. Effects of hydrophilic-lipophilic ability of surfactant, mixing ratio, and salinity of aqueous phase on synergism were investigated. It is found that the synergism for producing low interfacial tensions derives from the change of the hydrophilic-lipophilic abilities of the surfactant systems from the individual surfactants to their mixtures, which is confirmed by analyzing the alkane scanning curves, and the method to achieve synergism is adding the hydrophilic nonionic surfactant to the lipophilic alkylbenzene sulfonate, and ultralow interfacial tensions (<0.01 mN/m) can be obtained in proper mixing ratios. The results are of great significance for enhanced oil recovery.

Studies of Interfacial Activities of Four Kinds of Surfactants at Oil/Water Interface

This article aims to compare the interfacial activities of different kinds of surfactants in the same oil/water system. The anionic surfactants of alkylbenzene sulfonates, the polyoxyethylenated nonionic surfactants, the cationic surfactants of alkyl trimethyl ammonium chlorides, and the zwitterionic surfactants of alkyl hydroxyl sulfobetaines were used, and the interfacial tensions of the surfactant solutions against kerosene at different NaCl concentrations were measured. It is found that the interfacial activities of the alkylbenzene sulfonates are high and ultralow interfacial tensions (<0.01 mN/m) can be obtained at proper salinities. While, the nonionic surfactants have relatively low interfacial activities and the minimum tensions are around 0.01 mN/ms. The salinity scanning curves of the alkylbenzene sulfonates and nonionic surfactants decrease first, then increase, showing their interfacial activities can be changed by the salinity effectively. The cationic and zwitterionic surfactants have very low interfacial activities, of which all the tensions are higher than 0.1 mN/ms and are hard to be changed by the salinity. The experimental results may have important reference values for enhanced oil recovery.

A fractal basis for soil-water characteristics curves with hydraulic hysteresis

A soil-water characteristic curve with hydraulic hysteresis is derived using fractals by treating pores as either bodies or throats. As suction is increased along the main drying curve, drying of a body is controlled by the largest throat connected to it. As suction is reduced along the main wetting curve, the absorbed water collects in the smallest bodies and throats first, and then fills larger bodies and throats in order of size. The curve is fitted to data for a silt loam. The fit is reasonable for main drying and wetting curves, but less good for scanning curves.

A random solid‐porous model to simulate the retention curves of soils

SUMMARYThis paper describes a random solid‐porous model capable of simulating the structure of porous materials. To this purpose, the grain and pore size distributions as well as the void ratio of the material are required. Solids and pores are distributed at random in the model's space according to a size strategy. Herein, the model is used to simulate the retention curves of soils. The Laplace equation is used to determine the size of the pores able to saturate or dry during a wetting or drying process, respectively. The continuous path principle is used to define those elements that effectively saturate or dry during these processes. With this procedure, it is possible to simulate the main retention curves as well as the scanning curves during wetting–drying cycles. Some experimental results reported in the international literature have been used to test the model. This model can be enhanced to study the mechanical behavior of unsaturated soils. Copyright © 2012 John Wiley &amp; Sons, Ltd.

Effect of hydraulic hysteresis on seepage analysis for unsaturated soils

This paper describes a study into the effects of hysteresis on the hydraulic behaviour of unsaturated soil covers. The Richards equation is coupled with the hysteretic soil water characteristic curves (SWCCs), to describe water flow in unsaturated soils. A linear scanning curve on semi-logarithmic scales is used to describe the hydraulic behaviour within the main hysteretic loop in unsaturated seepage problems. The proposed approach is implemented for one-dimensional flow problems using the finite difference method. A comparison of predictions of the proposed hysteretic model with those of the traditional non-hysteretic model is carried out first. For the assumed scenario of a compacted clay cover, the non-hysteretic model using the main drying SWCC leads to significantly different results than the hysteretic model, further demonstrating the necessity of accounting for hysteresis in unsaturated seepage analysis. Moreover, three infiltration tests on layered soil columns are simulated with the proposed hysteretic model. Results show that hysteresis plays an important role in the seepage analysis of unsaturated soils subjected to cyclic drying and wetting. The hysteretic model is suggested for use in the design and evaluation of the hydraulic performance of soil covers.

Capillary Hysteresis Internal Variable Model of Unsaturated Soils

Soil-water characteristic curve describes the relationship between suction and volumetric water content of unsaturated soils. Air-enter have great influence on the drying scanning curves while the starting points of the scanning curve were at the highly saturated range. Based on capillary hysteresis internal variable model and conventional soil-water characteristics theory of unsaturated soils, a modified model which can simulate soil-water characteristic relationship that could consider the influence of air-enter is proposed. The influence of the air-enter on the drying scanning curves while the starting points were at the highly saturated range could be considered while this model is used. Numerical calculations are carried out on several kinds of soils. The simulating results show that the modified model is able to simulate the drying scanning curves while the starting points of the scanning curve were at the highly saturated range better than the capillary hysteresis internal variable model does.

Electrodeposition and corrosion behavior of nanostructured Ni-TiN composite films

The Ni-TiN nanocomposite film was successfully electrodeposited on brass copper substrates. The microstructures of the Ni-TiN nanocomposite film were investigated using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Its average grain size was analyzed through X-ray diffraction (XRD), and its anti-corrosion property was studied through potentiodynamic scanning curves and electrochemical impedance spectroscopy (EIS). The results show that the morphology of Ni-TiN composite film is sensitively dependent on the electroplating current density, TiN nanoparticle concentration, solution stirring speed, bath temperature and pH value of solution. The average grain size of the optimized nanocomposite film is about 50 nm. Meanwhile, the Ni-TiN nanocomposite films are much more resistant to corrosion than pure Ni coatings.

A Model for the Soil-Water Characteristic Curve and its Application in Dam Engineering

A mathematical model for the soil-water characteristic curve is proposed in the light of bounding surface plasticity. The main drying and wetting curves are taken as the asymptotes of the scanning curves, and only one additional parameter is introduced to simulate such scanning curves. To pave to the way for the application of the proposed model, the governing equation of unsaturated seepage problems and the finite element formulations are derived. A FEM program incorporating the SWCC model is then developed and used to study the hydraulic behaviour of an earth dam undergoes a repeated change of reservoir level. Numerical results confirm the possibility and necessity of using such a hysteresis model in unsaturated seepage problems.

Field observation of the response to pumping and recovery in the water table region of an unconfined aquifer

Summary A 24-h pumping test was conducted in the shallow water table portion of an unconfined aquifer located at Canadian Forces Base Borden near Alliston, Ontario, Canada. This test was designed to monitor the dynamic nature of the vertical gradients that form within the zone of tension saturation above the water table during pumping and recovery. During the test, pressure head was monitored throughout the saturated zone, both above (tensiometers) and below (piezometers) the water table; soil water content was monitored using both neutron moisture probes and time domain reflectometry (TDR). Following pumping, recovery was monitored in the same manner as the pumping portion of the test. The hydraulic head drawdown observed above the water table in the tension saturated zone during pumping and recovery was very similar to the drawdown observed below the water table. Vertical gradients throughout the saturated zone appeared to be uniform and low, and did not change significantly in the transition across the water table. The magnitude of the vertical gradients peaked early in the test, and subsequently decreased, with the reduction being more significant at greater radial distances from the pumping well. Neutron measurements indicate that only a minor degree of drainage occurred within the measurement interval for the duration of the test, a result of the low magnitude of drawdown generated during pumping and influence of hysteresis on the moisture profile. Although drainage was negligible from within the tension saturated zone during pumping, the hydraulic head drawdown below the water table displayed reduced rates of drawdown during the intermediate period of the test. TDR data was only collected during recovery; however, the data set displays hysteretic behavior in the pressure–saturation profiles, with all profiles plotting on scanning curves. Based on these observations it appears that a conceptual model in which vertical gradients below the water table are of the same order of magnitude as those within the capillary fringe is appropriate for the analysis of vadose zone response to pumping.