PADA: Power-aware development assistant for mobile sensing applications

� 2016 ACM. We propose PADA, a new power evaluation tool to measure and optimize power use of mobile sensing applications. Our motivational study with 53 professional developers shows they face huge challenges in meeting power requirements. The key challenges are from the significant time and effort for repetitive power measurements since the power use of sensing applications needs to be evaluated under various real-world usage scenarios and sensing parameters. PADA enables developers to obtain enriched power information under diverse usage scenarios in development environments without deploying and testing applications on real phones in real-life situations. We conducted two user studies with 19 developers to evaluate the usability of PADA. We show that developers benefit from using PADA in the implementation and power tuning of mobile sensing applications.N

Electrocatalytic arsenite oxidation in bicarbonate solutions combined with CO₂ reduction to formate

Sunlight-driven water-energy nexus technologies are receiving increasing attention. This study presents a hybrid electrochemical system that catalyzes the oxidation of As(III) to As(V) with a nanoparticulate TiO₂ electrocatalyst (Ti/Ir_(1-x)Ta_xO_y/TiO₂; denoted as an n-TEC) while simultaneously converting CO₂ to formate on a Bi electrode in aqueous bicarbonate solutions at circum-neutral pH. Linear sweep voltammograms of n-TEC exhibit a specific As(III) oxidation peak (E_(p,As)), at which the Faradaic efficiency (FE) of As(V) production is ∼100%. However, the application of a potential higher than the peak (E > E_(p,As)) leads to a significant decrease in the FE due to water oxidation. Upon the addition of chloride, the oxidation of water and chloride occur competitively, producing reactive chlorine species responsible for mediating the oxidation of As(III). The Bi electrodes synthesized via the electrodeposition of Bi³⁺ typically show high FEs of >80% for formate production in bicarbonate solution purged with CO₂. The addition of chloride significantly enhances the current while maintaining the FE. The n-TEC catalyst and Bi electrodes are paired in a single device equipped with a membrane, and significant effort is made to achieve the same FEs in both the anodic and cathodic reactions as in their half-reactions. Finally, the optimized n-TEC/Bi pair is coupled with a low-cost, commercially available photovoltaic (PV). Various technical factors that drive the overall reactions with the PV are considered, and maximum FEs of ∼95% are achieved for the production of both As(V) and formate

An experimental and thermodynamic study on CH4-CO2 replacement mechanism in various clathrate structures and depressurization-assisted replacement for effective CH4 production and CO2 sequestration

Department of Urban and Environmental Engineering (Environmental Science and Engineering)Natural Gas Hydrates (NGHs) have been regarded as a future clean energy resource because of the enormous deposit of it in the earth. The development of NGHs could be a solution to fulfill the demands for natural gas that are constantly increasing to reduce CO2 emission. Among the known methods for the production of natural gas from NGHs, the replacement method has been suggested as a promising technology because it could achieve carbon-neutral energy production via swapping of CH4 in NGHs with CO2. Three different structures of NGHs (sI, sII, and sH) have been discovered through several explorations. Distinctive thermodynamic characteristics of each structure required the study for the precise replacement mechanism in various structures of NGHs, but previous studies for replacement were mainly focused on sI hydrate. This study not only investigated the influence of flue gas injection in various hydrate reservoirs to reveal replacement mechanisms but also developed the depressurization-assisted replacement for enhancing the economic feasibility of the replacement. To verify the replacement mechanism of the flue gas injection into various structures of gas hydrate, sII CH4 + C3H8 and sH CH4 + methylcyclopentane hydrate were used in this study. The influence of feed gas composition was investigated with three mixture gases (CO2 (20%) + N2 (80%), CO2 (40%) + N2 (60%), and CO2 (60%) and N2 (40%)) to enhance the replacement efficiency. The extent of replacement depending on the pressure and compositions of feed gas were measured to compare the efficiency and kinetics of guest exchange. The structure transition and cage-specific guest distribution before and after replacement were examined by using 13C nuclear magnetic resonance (13C NMR) and powder X-ray diffraction (PXRD). In contrast to sII hydrate - CO2 replacement, N2 inclusion occurred in sII hydrate-flue gas replacement resulted in iso-structural replacement and a lower replacement efficiency than expected. The competitive inclusion of CO2 and N2 occurred in small (512) cages of sII hydrate, resulted in lower efficiency than pure CO2 injection. Furthermore, the mechanism of lattice expansion in sII hydrate-flue gas replacement was revealed by using molecular dynamics simulation in this study. In sH hydrate-flue gas replacement, partial structure transition was observed depending on the composition and pressure of the feed gas, it not only accelerated the replacement but enhanced the efficiency. Although the higher CO2 composition in the feed gas enhanced the structure transition, it also reduced the inclusion of N2 by the decreased partial pressure of N2 in feed gas resulted in the low extent of replacement. In addition, the depressurization-assisted replacement was investigated to overcome the low productivity of replacement. As a preliminary study, an effective driving force for dissociation of gas hydrate was investigated to efficiently control initial depressurization. The modified chemical potential-based driving force was revealed to the most optimal driving force for controlling depressurization without relevance of the reservoir temperature via various production tests in one-dimensional sediment packing reactor. Although partial dissociation of gas hydrate occurred in the early period of depressurization-assisted replacement, instant re-formation of gas hydrate was observed after CO2 injection which could assure the geological safety of hydrate-bearing sediment. A remarkable enhancement of replacement efficiency was observed as increasing the dissociation ratio of initial gas hydrate. Furthermore, the result of this study demonstrated that a fast flow rate of CO2 could enhance the productivity of depressurization-assisted replacement by reducing CH4 re-enclathation. The overall experimental results not only provided valuable insights for a comprehensive understanding of replacement mechanism in various NGHs but also demonstrated the technical feasibility of depressurization-assisted replacement which could be a breakthrough for efficient CH4 recovery and CO2 sequestration in the replacement.ope

Time-dependent observation of a cage-specific guest exchange in sI hydrates for CH4 recovery and CO2 sequestration

CH4-CO2 replacement in naturally occurring gas hydrates has been considered a promising method for both energy recovery and CO2 sequestration. In this study, the time-dependent guest exchange behaviors and guest distributions during CH4 -CO2 replacement were closely examined at two different CO2 injecting pressures (2.2 and 3.5 MPa) using nuclear magnetic resonance (NMR), in-situ Raman spectroscopy, powder X-ray diffraction (PXRD), and gas chromatography. The C-13 NMR spectra confirmed that the cage occupancy ratio of the CH4 molecules in the large 5(12)6(2) and small 5(12) cages (theta(L)/theta(S,CH4)) after the replacement was significantly smaller than that before the replacement because of the preferential occupation of CO2 in the large 5(12)6(2) cages. The time-dependent Raman spectra revealed that the rate of CO2 inclusion and the resultant CH4 depletion in the hydrate phase during the replacement was faster at a higher CO2 injecting pressure. The Rietveld refinement of the PXRD patterns offered a quantitative cage occupancy of CH4 and CO2 molecules before and after the replacement. The time-dependent cage occupancy values of CH4 and CO2 during the replacement obtained from a mull-methodological approach, which is a combination of PXRD analysis and in-situ Raman measurement, demonstrated that a significant guest exchange in the large 5(12)6(2) cages had a greater effect on the extent of replacement and that the kinetics of the CH4-CO2 replacement was accelerated at a higher CO2 injecting pressure. The results provide a better understanding of the kinetics and mechanism of the cage-specific CH4-CO2 replacement occurring in the sI hydrates for CH4 recovery and CO2 sequestration

Theoretically achievable efficiency of hydrate-based desalination and its significance for evaluating kinetic desalination performance of gaseous hydrate formers

Freshwater can be obtained through gas hydrate formation from saline water, which is called hydrate-based desalination (HBD). In this study, the thermodynamic and kinetic features of propane, R134a, R22, and R152a hydrates in the presence of NaCl were examined to determine the energy-efficient gaseous hydrate former. Structure I hydrate formers (R22 and R152) gave lower hydrate depression temperatures than structure II hydrate formers (propane and R134a) at a given salinity. The theoretically achievable salinity and water yield of HBD at each given thermodynamic condition were calculated using the Hu-Lee-Sum correlation. The theoretical HBD efficiency increased as the initial salinity decreased, the operating pressure decreased, and the initial subcooling temperature increased. At a fixed initial subcooling (2 K), R134a gave faster formation kinetics in the early stage, but R22 eventually offered highest hydrate conversion. At a fixed temperature (272 K), R152a showed fastest formation kinetics and highest HBD efficiency due to its milder hydrate equilibrium conditions. The overall results demonstrated that the thermodynamic stability and the intrinsic formation kinetics of gas hydrates significantly impact HBD efficiency and that the theoretical HBD efficiency can be the quantitative criterion for evaluating the kinetic performance and desalination capability of hydrate formers

Evaluation of kinetic salt-en ric hment behavior and separation performance of HFC-152a hydrate-based desalination using an experimental measurement and a thermodynamic correlation

Hydrate-based desalination (HBD), a type of freezing-based desalination, can concentrate salts of saline water and produce fresh water via hydrate crystal formation. In this study, the thermodynamic stability, crystallographic information, and kinetic growth behavior of HFC-152a hydrate were investigated to estimate the desalination efficiency of HBD. The phase equilibria revealed that the HFC-152a hydrate could be formed at a higher temperature in the presence of NaCl (0 wt%, 3.5 wt%, and 8.0 wt%) than the HFC-134a hydrate at 0.3 MPa. The hydration number of the HFC-152a hydrate (sI) was found to be 7.74 through the Rietveld refinement of the powder X-ray diffraction patterns, and it was also used to determine the dissociation enthalpy of the HFC-152a hydrate. The Hu-Lee-Sum correlation was employed to predict the equilibrium shift and hydrate depression temperature of both HFC-152a and HFC-134a hydrates in the presence of NaCl. Faster hydrate growth kinetics and higher hydrate conversion were observed for the HFC-152a hydrate in saline solutions despite the smaller initial driving force at 0.3 MPa and the subcooling temperature of 3 K. Additionally, to quantify the desalination efficiency of the HFC-152a HBD, the maximum achievable salinity and maximum water yield were examined using the HLS correlation. The salt-enrichment efficiency decreased with an increase in the initial salinity and increased with increasing the subcooling. The overall results indicate that HFC-152a is, potentially, a superior candidate for HBD. The novel approach examined in this study will be useful for assessing the desalination efficiency of the HBD process. (c) 2021 Elsevier Ltd. All rights reserved

The dual-functional roles of N-2 gas for the exploitation of natural gas hydrates: An inhibitor for dissociation and an external guest for replacement

In this study, the dissociation behavior and the time-dependent guest distributions of methane (CH4) hydrate after the injection of gaseous N-2 were closely investigated using gas chromatography, in situ Raman spectroscopy, and nuclear magnetic resonance (NMR) spectroscopy for two different N-2 gas injecting pressures (4.0 and 8.0 MPa) at three different temperatures (268.8, 274.2, and 278.2 K). The dissociation kinetics of the CH4 hydrate were accelerated at a higher temperature and a lower N-2 injecting pressure. Time-dependent Raman spectra confirmed that the N-2 molecules began to be captured in the hydrate cages immediately after the N-2 gas injection at 268.8 K, and the extent of N-2 incorporation in the hydrate phase increased at a higher N-2 gas injecting pressure. The C-13 NMR spectra revealed that an increase in the N-2 composition in the hydrate phase induced a structural transition of the CH4 + N-2 hydrates from sI to sII and that N-2 molecules were preferentially captured in the small (512) cages of the CH4 + N-2 hydrates. The overall results provide deep insight into the exact role of N-2 mol-ecules in the inhibitor injection method and CH4 - CO2 + N-2 replacement methods for the exploitation of natural gas hydrates. (C) 2021 Elsevier Ltd. All rights reserved

Clathrate-Based CO2 Capture from Co-2-Rich Natural Gas and Biogas

In this study, clathrate-based CO2 capture was investigated in the presence of thermodynamic promoters such as tetrahydrofuran (THF) and tetra-n-butyl ammonium chloride (TBAC) for upgrading CO2-rich natural gas and biogas. The phase equilibria, gas uptakes, gas composition measurements, and spectroscopic analyses of CH4 (50%), CO2 (50%), and promoter clathrates were examined with a primary focus on the effects of thermodynamic promoters on clathrate stability and cage filling behavior. The addition of THF and TBAC significantly enhanced the thermodynamic stability of CH4 (50%) and CO2 (50%) clathrates. C-13 NMR and Raman spectroscopy clearly revealed that CO2 and CH4 are enclathrated in the clathrate cages. THF solutions demonstrated a faster growth rate of clathrates, but CO2 was less selective than CH4 in the THF clathrate phase due to the lower thermodynamic stability of the CO2 and THF clathrate compared to the CH4 and THF clathrate. TBAC solutions produced higher CO2 selectivity in the semiclathrate phase due to the presence of distorted small cages, which have a strong preference for CO2 molecules. The experimental results demonstrated that CO2 selectivity in the clathrate phase can be influenced by the thermodynamic stability, cage shape and dimension, and cage filling behavior in the presence of thermodynamic promoters, and thus, a suitable promoter and their optimum concentration should be carefully determined in designing and operating clathrate-based CO2 capture from natural gas or biogas

Effects of pressure and temperature conditions on thermodynamic and kinetic guest exchange behaviors of CH4 - CO2 + N-2 replacement for energy recovery and greenhouse gas storage

Both natural gas production and CO2 sequestration can be simultaneously achieved in natural gas hydrates (NGHs) by using a guest swapping technique. In this study, the effects of replacement pressure (10.0-18.5 MPa) and temperature (274.2-277.2 K) conditions on the guest exchange behaviors of CH4 - CO2 + N-2 replacement were investigated, focusing on the extent of replacement and replacement kinetics. At 274.2 K, the extent of replacement increased with injection pressure of CO2 (20%) + N-2 (80%) gas, which is mainly attributed to a larger N-2 inclusion at a higher pressure. At a higher temperature, the extent of replacement did not change, but CO2/N-2 ratio in the replaced hydrates decreased slightly. An increase in the pressure led to an accelerated CO2 inclusion rate in the large (5(12)6(2)) cages at the initial stage and an enhanced N-2 inclusion in the small (5(12)) cages at the final stage. The enhanced replacement kinetics at a higher temperature is attributable to the increased inclusion rates of both CO2 and N-2 at the initial stage of replacement. The results provide valuable insights into the guest swapping mechanism of CH4 - CO2 + N-2 replacement occurring in NGH reservoirs with various locations and environments. (C) 2021 Elsevier Ltd. All rights reserved

Kinetic CO 2 selectivity in clathrate-based CO 2 capture for upgrading CO 2 -rich natural gas and biogas

Upgrading CO 2 -rich natural gas or biogas through CO 2 capture is essential to reduce greenhouse gas emissions and to increase its energy density. In this study, clathrate-based CO 2 capture from CO 2 -rich natural gas or biogas was investigated with a primary focus on kinetic CO 2 selectivity. The time-dependent CO 2 selectivity during clathrate formation for pure water, tetrahydrofuran (THF, 5.6 mol%) solution, and tetra-n-butylammonium chloride (TBAC, 3.3 mol%) solution was examined through direct composition analysis and in situ Raman spectroscopy. In pure water, the CO 2 composition in the clathrate phase was much higher at the early stage of clathrate formation than that at equilibrium, indicating that CO 2 is kinetically and thermodynamically selective. For both the THF (5.6 mol%) and TBAC (3.3 mol%) solutions, the CO 2 composition in the clathrate phase was almost constant during clathrate formation. However, the TBAC (3.3 mol%) solution showed significantly higher CO 2 composition (???74%) throughout the reaction, whereas the THF (5.6 mol%) solution exhibited enrichment of CH 4 in the clathrate phase. The experimental results clearly demonstrate that CO 2 selectivity is dependent on both kinetics and equilibrium of clathrate hydrates and that the addition of thermodynamic promoters, such as THF and TBAC, can affect kinetic CO 2 selectivity as well as equilibrium CO 2 selectivity in the clathrate phase