Dynamic Dissociation Behaviors of sII Hydrates in Liquid Water by Heating: A Molecular Dynamics Simulation Approach

An understanding of the dynamic behavior of subtle hydrate dissociation in the liquid water phase is fundamental for gas production from marine hydrate reservoirs. Molecular dynamics simulations are performed in this study to investigate the dissociation kinetics of pure propane and binary propane + methane sII hydrates in a liquid water environment. The results show that faster hydrate dissociation rates are observed at higher initial temperatures. The hydrate phase dissociates from the cluster surface to the inside in a layer-by-layer manner under the simulation temperature conditions, which is similar to the behavior of sI hydrates and is independent of the hydrate crystal type. Compared to the binary sII hydrate, the pure sII hydrate dissociates more easily under the same initial temperature conditions, which can be attributed to the stabilizing effect of guest molecules in the hydrate cages. The empty cages collapse in one step, in contrast to the two-step pathway induced by the guest-host interaction. In addition, a hydrocarbon phase forms in the binary hydrate dissociation system instead of nanobubbles. These results can provide molecular-level insights into the dynamic mechanism of hydrate dissociation and theoretical guidance for gas recovery by thermal injection from marine hydrate reservoirs. Engineering Thermodynamic

Microscopic insights into poly- and mono-crystalline methane hydrate dissociation in Na-montmorillonite pores at static and dynamic fluid conditions

Knowledge on the kinetics of gas hydrate dissociation in clay pores at static and dynamic fluid conditions is a fundamental scientific issue for improving gas production efficiency from hydrate deposits using thermal stimulation and depressurization respectively. Here, molecular dynamics simulations were used to investigate poly- and mono-crystalline methane hydrates in Na-montmorillonite clay nanopores. Simulation results show that hydrate dissociation is highly sensitive to temperature and pressure gradients, but their effects differ. Temperature changes increase thermal instability of water and gas molecules, leading to layer-by-layer dissociation from the outer surface. Under flow conditions, laminar flow predominates in nano-pores, and non-Darcy flow occurs due to clay-fluid interactions. Viscous flow disrupts hydrogen bonding at the hydrate surface, enhancing kinetic instability of water. Grain boundaries of polycrystalline hydrates are less stable compared to bulk phases and preferentially decompose, forming new dissociation fronts. This accelerates dissociation compared to monocrystalline hydrates. Fracture occurs at the grain boundaries of polycrystalline hydrate in the fluid, resulting in separate hydrate crystal grains. This fracture process further accelerates hydrate dissociation. In flow systems, methane nanobubbles form in fluid and readily transport with fluid flow. Unlike surface nanobubbles at static conditions, these liquid nanobubbles exhibit mobility. The findings of this study can contribute to a better understanding of the complex phase transition behavior of hydrate in confined environment, and provide theoretical support for improving production control technology.Green Open Access added to TU Delft Institutional Repository 'You share, we take care!' - Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Engineering Thermodynamic

Numerical study of response behaviors of natural gas hydrate reservoir around wellbore induced by water jet slotting

The trial production of natural gas hydrate reservoirs remains poor. Reasonable reservoir reconstruction, which can improve formation permeability, is an important approach to increasing the efficiency and enhancing production. In this work, water jet slotting is proposed to reconstruct an natural gas hydrate reservoir near a wellbore. The spatial slots formed by water jet slotting not only directly constitute high-permeability channels, but also generate disturbances to the surrounding in-situ sediment. Water jet slotting disturbances to nearby sediment was investigated using a three dimensional flow-structure coupling model to evaluate the proposed reconstruction method. The reservoir at the SH2 site in the Shenhu area of the South China Sea was used as the reference. A horizontal slotting arrangement along the vertical well was adopted. The results demonstrate that water jet slotting can change the primary stress state of the sediment around the wellbore, and generate a dominant stress relaxation zone and small stress concentration zone. Within the stress relaxation zone, the in-situ compressive stress was remarkably reduced or even transformed into tensile stress, accompanied by sediment displacement and volumetric expansion strain. This is conducive to loosening the sediment around the wellbore and improving the permeability characteristics. In addition, the influence of the water jet slotting parameters including slot radius, spacing, and number on disturbances to the nearby sediment was studied. Reservoir responses to water jet slotting under balanced and unbalanced bottom-hole pressures were compared and analyzed. This study provides a reference for natural gas hydrate reservoir reconstruction using water jet slotting.Engineering Thermodynamic

Fresh and hardened properties of self-compacting concrete containing recycled fine clay brick aggregates

Clay brick is one of the major components of demolition waste, which is generally landfilled. Effective and new uses of recycled clay brick may provide sustainability benefits in terms of landfill reduction. Therefore, this research aims at applying Recycled fine clay brick aggregates (RFCBA) with sizes from 0.075 mm–4.75 mm to prepare Self-compacting concrete (SCC). The effects of RFCBA on fresh and hardened properties of SCC were investigated. Saturated surface dry RFCBA was used to replace Natural fine aggregate (NFA) with the percentage of 25%, 50%, 75% and 100%, respectively, in making the SCC mixes. Although experimental results showed that the flowability, passing ability, and segregation resistance of SCC containing RFCBA (RFCBA-SCC) decreased with the increasing RFCBA content, these properties still satisfy the criteria of SCC. The compressive strength, splitting strength, flexural strength, and elastic modulus of the RFCBA-SCC mixes decreased with an increase of RFCBA content. Due to their porous nature, recycled fine clay brick aggregates may also be a source of additional water for internal curing. The internal curing effect was confirmed by the mercury intrusion porosimetry, X-ray diffraction, and thermogravimetric analysis measurements. Moreover, a significant autogenous shrinkage reduction of SCC is achieved by using the RFCBA due to the release of additional water pre-stored in the RFCBA. Therefore, it can be concluded that the addition of RFCBA to SCC mixtures can provide additional practical benefits in the hardened state.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work publicMaterials and Environmen

Negative Effects of Inorganic Salt Invasion on the Dissociation Kinetics of Silica-Confined Gas Hydrate via Thermal Stimulation

Methane hydrate dissociation kinetics can be inhibited in NaCl solutions; however, this effect is reversed by promoting bubble formation that enhances dissociation. The negative and positive effects of inorganic salt injection on gas production from hydrate-bearing sediments are still controversial. Here, molecular dynamics simulations were performed to investigate the characteristics of NaCl solution invasion into hydrate-occupied nanopores and the effects on the confined hydrate dissociation kinetics. Two initial configurations comprising liquid and silica pore phases were studied with a low or high NaCl concentration, respectively. The results show that, under the simulation conditions, salt invasion decelerated hydrate dissociation within the silica pore as NaCl invasion into the pore is stepwise. Initially, few ions can diffuse into the pore phase, and gas nanobubbles form on the solid surface mainly via confinement and surface effects, independent of NaCl solution invasion. Subsequently, gradual salt diffusion immersed the residual hydrate in the salt solution and hindered hydrate decomposition until the dissociation finished. More ions could diffuse into the pore phase at the high NaCl concentrations with a low diffusion efficiency, leading to surface nanobubble growth toward the residual hydrate and somewhat accelerated hydrate dissociation. This severely hinders the escape of released methane from the pore. This study yields molecular-level insight into the origin of the negative effect of salt invasion on hydrate dissociation, which should be avoided during gas production from hydrate reservoirs with low permeabilities via salt injection combined with thermal stimulation. Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Engineering Thermodynamic

Context-aware stated preferences with smartphone-based travel surveys

Stated preferences surveys are most commonly used to provide behavioral insights on hypothetical travel scenarios such as new transportation services or attribute ranges beyond those observed in existing conditions. When designing SP surveys, considerable care is needed to balance the statistical objectives with the realism of the experiment. This paper presents an innovative method for smartphone-based stated preferences (SP) surveys leveraging state-of-the-art smartphone-based survey platforms and their revealed preferences sensing capabilities. A random experimental design generates context-aware SP profiles using user specific socioeconomic characteristics and past travel data along with relevant web data for scenario generation. The generated choice tasks are automatically validated to reduce the number of dominant or inferior alternatives in real-time, then validated using Monte-Carlo simulations offline. In this paper we focus our attention on mode choice and design an experiment that considers a wide range of possible existing mode alternatives along with a new alternative on-demand mobility service that does not exist in real life. This experiment is then used to collect SP data or a sample of 224 respondents in the Greater Boston Area. A discrete mode choice model is estimated to illustrate the benefit of the proposed method in capturing current context-specific preferences in response to the new scenario.Green Open Access added to TU Delft Institutional Repository 'You share, we take care!' - Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Transport Engineering and Logistic

Building water quality deterioration during water supply restoration after interruption: Influences of premise plumbing configuration

Premise plumbing plays an essential role in determining the final quality of drinking water consumed by customers. However, little is known about the influences of plumbing configuration on water quality changes. This study selected parallel premise plumbing in the same building with different configurations, i.e., laboratory and toilet plumbing. Water quality deteriorations induced by premise plumbing under regular and interrupted water supply were investigated. The results showed that most of the water quality parameters did not vary under regular supply, except Zn, which was significantly increased by laboratory plumbing (78.2 to 260.7 µg/l). For the bacterial community, the Chao1 index was significantly increased by both plumbing types to a similar level (52 to 104). Laboratory plumbing significantly changed the bacterial community, but toilet plumbing did not. Remarkably, water supply interruption/restoration led to serious water quality deterioration in both plumbing types but resulted in different changes. Physiochemically, discoloration was observed only in laboratory plumbing, along with sharp increases in Mn and Zn. Microbiologically, the increase in ATP was sharper in toilet plumbing than in laboratory plumbing. Some opportunistic pathogen-containing genera, e.g., Legionella spp. and Pseudomonas spp., were present in both plumbing types but only in disturbed samples. This study highlighted the esthetic, chemical, and microbiological risks associated with premise plumbing, for which system configuration plays an important role. Attention should be given to optimizing premise plumbing design for managing building water quality.Sanitary Engineerin

Scaling law of resolved-scale isotropic turbulence and its application in large-eddy simulation

Eddy-dampingquasinormal Markovian (EDQNM) theory is employed to calculate the resolved-scale spectrum and transfer spectrum, based on which we investigate the resolved-scale scaling law. Results show that the scaling law of the resolved-scale turbulence, which is affected by several factors, is far from that of the full-scale turbulence and should be corrected. These results are then applied to an existing subgrid model to improve its performance. A series of simulations are performed to verify the necessity of a fixed scaling law in the subgrid modeling

Crack instability of ferroelectric solids under alternative electric loading

The low fracture toughness of the widely used piezoelectric and ferroelectric materials in technological applications raises a big concern about their durability and safety. Up to now, the mechanisms of electric-field induced fatigue crack growth in those materials are not fully understood. Here we report experimental observations that alternative electric loading at high frequency or large amplitude gives rise to dramatic temperature rise at the crack tip of a ferroelectric solid. The temperature rise subsequently lowers the energy barrier of materials for domain switch in the vicinity of the crack tip, increases the stress intensity factor and leads to unstable crack propagation finally. In contrast, at low frequency or small amplitude, crack tip temperature increases mildly and saturates quickly, no crack growth is observed. Together with our theoretical analysis on the non-linear heat transfer at the crack tip, we constructed a safe operating area curve with respect to the frequency and amplitude of the electric field, and validated the safety map by experiments. The revealed mechanisms about how electro-thermal-mechanical coupling influences fracture can be directly used to guide the design and safety assessment of piezoelectric and ferroelectric devices. (C) 2015 Elsevier Ltd. All rights reserved