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This study proposes a reliable evaluation method for three-phase saturation (water, gas hydrate (GH), and gas) evaluation during the GH dissociation core experiment using deep learning. A convolutional neural network (CNN) takes computed tomography (CT) images obtained during the GH core experiment as an input and provides three-phase saturation as an output. Although machine/deep learning methods have been applied to the saturation evaluation from CT images in previous research, they were not reliable due to the lack of adequate amount of training data where the model could not find appropriate parameters. Besides, non-zero gas hydrate saturation showed where it was supposed to be zero. This study improved the evaluation of three-phase saturation and solved the non-zero GH saturation problem by acquirement of extra data and application of data augmentation with CNN. The results of CNN and CNN with data augmentation presented 34% and 29% error compared to those of random forest. CNN with data augmentation brought 85% and 44% of error and its variance compared to those of CNN without data augmentation, respectively. Consequently, based on domain knowledge for GH, when it comes to the robustness of random data composition and consistency of performance, the evaluation of three-phase saturation can be boosted using CNN with data augmentation.

We experimentally examine the impingement of a fully turbulent round jet (with diameter Dj) on a long circular cylinder (with diameter D) in the crossflow plane coinciding with the jet axis for D/Dj = 2.5 and ReDj = 20,000. Particular focus is placed on the kinematics of boundary layer transition that occurs and subsequently leads to a second thermal peak on the cylinder surface downstream of the primary thermal peak near the stagnation point when placed inside the jet’s potential core. To this end, spectral analyses of wall shear stress data and time-resolved velocity data within both laminar and turbulent boundary layers have been performed. The present study demonstrates that the root mean square (rms) fluctuation of the stream-wise velocity component increases along both boundary layers due to the propagation of external perturbations from coherent structures that are shed from the jet exit, which incites the transition of the laminar boundary layer. The transition causes the local heat transfer elevation that interrupts the monotonic decrease from the primary thermal peak whereas it plays no direct part in forming the second thermal peak. Instead, the second thermal peak occurs at a delayed downstream azimuth angle from the transition where the rms velocity fluctuations of the boundary layer flow reach their peak at the dominant frequency, equivalent to that of the coherent structures of the jet.

This study investigated a structural transition induced by cage-dependent guest exchange in the CH4 + C3H8 hydrate with CO2 injection for CH4 recovery and CO2 sequestration. The influence of the CO2 replacement on the crystalline structure of initial CH4 + C3H8 hydrates and the cage-dependent distribution of guest molecules were quantitatively investigated using powder X-ray diffraction, 13C nuclear magnetic resonance spectroscopy, and gas chromatography. The quantitative analyses demonstrated that the CO2 occupation caused the depletion of C3H8 molecules in the large 51264 cages of structure II hydrates, thereby resulting in the subsequent transformation into CO2-rich sI hydrates and the coexistence of structure I and structure II hydrates after the replacement. The guest-exchange behavior observed from time-dependent Raman spectra indicated that the replacement rate was increased with an increase in pressure of injected CO2 and that the extent of the replacement was enhanced at higher pressure of injected CO2. Overall experimental evidence of the partial structural-transition replacement suggests that CO2 molecules first occupied structure II hydrates predominantly with the rapid guest exchange at the surface and that the initial structure II hydrates were subsequently converted to the CO2-rich structure I hydrates from the surface to the inner side. Precise identification of the mechanism responsible for the partial structural transition occurring in the CH4 + C3H8 - CO2 replacement will be very helpful in developing a strategy for actual CO2 injection into structure II gas hydrate reservoirs for energy recovery and CO2 sequestration.

The CH 4 -flue gas replacement that occurs in structure H (sH) hydrates was investigated, with a primary focus on thermodynamic phase behavior, structure identification, heat flow monitoring, and dissociation enthalpy, for its dual function of CH 4 recovery and CO 2 sequestration. The significant shift in the equilibrium line of the CH 4 + neohexane (2,2-dimethylbutane, NH) hydrates replaced with CO 2 (10%) + N 2 (90%) and CO 2 (20%) + N 2 (80%) indicates that CH 4 molecules in the sH hydrates were significantly exchanged with flue gas mixtures. Furthermore, 13 C NMR spectroscopy revealed that replacement using flue gas occurs without a structural transition, and that CO 2 molecules preferentially attack CH 4 molecules occupied in the medium 4 3 5 6 6 3 cages of sH hydrates. The extent of the CH 4 + NH – flue gas replacement was approximately 74%. During CH 4 – flue gas replacement in sH hydrates, there was no significant change in heat flow associated with the dissociation and subsequent reformation of gas hydrates. Dissociation enthalpies of gas hydrates before and after replacement, measured using a high-pressure micro-differential scanning calorimeter (HP μ -DSC), also supported isostructural replacement with the high extent of reaction. This study reports the first experimental evidence of isostructural CH 4 – flue gas replacement occurring in sH hydrates, and thus, might contribute to extending the potential fields of CH 4 exploitation using a flue gas replacement, into sH natural gas hydrate reservoirs.

A replacement technique has been regarded as a promising strategy for both CH4 exploitation from gas hydrates and CO2 sequestration into deep-ocean reservoirs. Most research has been focused on replacement reactions that occur in sI hydrates due to their prevalence in natural gas hydrates. However, sII hydrates in nature have been also discovered in some regions, and the replacement mechanism in sII hydrates significantly differs from that in sI hydrates. In this study, we have intensively investigated the replacement reaction of sII (C3H8 + CH4) hydrate by externally injecting CO2/N2 (50:50) gas mixture with a primary focus on powder X-ray diffraction, Raman spectroscopy, NMR spectroscopy, and gas chromatography analyses. In particular, it was firstly confirmed that there was no structural transformation during the replacement of C3H8 + CH4 hydrate with CO2/N2 gas injection, indicating that sII hydrate decomposition followed by sI hydrate formation did not occur. Furthermore, the cage-specific replacement pattern of the C3H8 + CH4 hydrate revealed that CH4 replacement with N2 in the small cages of sII was more significant than C3H8 replacement with CO2 in the large cages of sII. The total extent of the replacement for the C3H8 + CH4 hydrate was cross-checked by NMR and GC analyses and found to be approximately 54%. Compared to the replacement for CH4 hydrate with CO2/N2 gas, the lower extent of the replacement for the C3H8 + CH4 hydrate with CO2/N2 gas was attributable to the persistent presence of C3H8 in the large cages and the lower content of N2 in the feed gas. The structural sustainability and cage-specific replacement observed in the C3H8 + CH4 hydrate with external CO2/N2 gas will have significant implications for suggesting target gas hydrate reservoirs and understanding the precise nature of guest exchange in gas hydrates for both safe natural gas production and long-term CO2 sequestration.

Purpose: This study aimed to compare the possible standard-setting methods for the Korean Radiological Technologist Licensing Examination, which has a fixed cut score, and to suggest the most appropriate method. Methods: Six radiological technology professors set standards for 250 items on the Korean Radiological Technologist Licensing Examination administered in December 2016 using the Angoff, Ebel, bookmark, and Hofstee methods. Results: With a maximum percentile score of 100, the cut score for the examination was 71.27 using the Angoff method, 62.2 using the Ebel method, 64.49 using the bookmark method, and 62 using the Hofstee method. Based on the Hofstee method, an acceptable cut score for the examination would be between 52.83 and 70, but the cut score was 71.27 using the Angoff method. Conclusion: The above results suggest that the best standard-setting method to determine the cut score would be a panel discussion with the modified Angoff or Ebel method, with verification of the rated results by the Hofstee method. Since no standard-setting method has yet been adopted for the Korean Radiological Technologist Licensing Examination, this study will be able to provide practical guidance for introducing a standard-setting process.

Breaking the bounds of classical natural gas hydrate (NGH) production processes, a newborn concept based on the gas exchange mechanism provides an opportunity to catch two birds with one stone: simultaneously achieving the sequestration of CO 2 for climate change mitigation and the enhanced recovery of CH 4 for energy production. As a ‘new paradigm’ in NGH production schemes, the non-destructive gas exchange as one of the most stable and promising NGH recovery approaches has received much attention in the fields of physics, chemistry, chemical engineering, civil engineering, petroleum engineering and geology. In this review, we assess the state-of-the-art gas exchange concept for NGH production by understanding its principles and developments, with emphasis on another technical breakthrough using the CO 2 + N 2 gas mixture injection. After establishing the fundamentals of the gas exchange process, we make a general survey of the NGH field production in the North Slope of Alaska in 2012, which practically adopted the gas exchange as a key technology. Several recent international NGH field production tests that basically use depressurization are also briefly analyzed for comparison. We suggest that the gas exchange method is ready to be tested in the NGH deposits with the valuable lessons learned from past pioneering tests.

The CH 4 –flue gas replacement in naturally occurring gas hydrates has attracted significant attention due to its potential as a method of exploitation of clean energy and sequestration of CO 2 . In the replacement process, the thermodynamic and structural properties of the mixed gas hydrates are critical factors to predict the heat flow in the hydrate-bearing sediments and the heat required for hydrate dissociation, and to evaluate the CO 2 storage capacity of hydrate reservoirs. In this study, the 13 C NMR and gas composition analyses confirmed that the preferential enclathration of N 2 molecules in small 5 12 cages of structure I hydrates improved the extent of the CH 4 recovery. A high pressure micro-differential scanning calorimeter (HP μ -DSC) provided reliable hydrate stability conditions and heat of dissociation values in the porous silica gels after the replacement, which confirmed that CH 4 in the hydrates was successfully replaced with flue gas. A heat flow change associated with the dissociation and formation of hydrates was not noticeable during the CH 4 –flue gas replacement. Therefore, this study reveals that CH 4 –flue gas swapping occurs without structural transitions and significant hydrate dissociations.

In this study, the mixed gas hydrates formed from the flue gas mixtures of CO 2 + N 2 have been investigated with a primary focus on the structure identification and the dissociation enthalpy measurements. The stability conditions of the CO 2 + N 2 gas hydrates are determined using an isochoric (PVT) method and a differential scanning calorimeter (DSC). It is found from the comparison of the hydrate phase equilibrium data measured using two methods that the DSC can be effectively used as an alternative method for measuring the stability conditions of the CO 2 + N 2 gas hydrates. The microscopic analyses, such as powder X-ray diffraction and Raman spectroscopy, demonstrated that the gas mixtures of CO 2 + N 2 form a structure I hydrate and that the structural transition does not occur in the range of the flue gas composition. To reveal the dissociation behavior of the mixed gas hydrates, the dissociation enthalpies of the CO 2 + N 2 gas hydrates have been measured using a micro-differential scanning calorimeter (μ-DSC). The dissociation heats of the CO 2 + N 2 gas hydrates increased with an increase of the CO 2 composition in the hydrate phase. The experimental results obtained in this study provide the thermodynamic and physical background required to estimate the heat liberated or absorbed during hydrate formation and dissociation and to predict the operation conditions for the gas hydrate-based CO 2 capture and storage process.

가스 하이드레이트는 고압 및 저온의 조건에서 주체 분자인 물의 수소 결합에 의해 형성된 격자 내로 저분자량의 기체 분자가 포집되어 있는 고체의 함유 화합물이다. 심해저 퇴적층과 영구 동토지역에 매장되어 있는 막대한 양의 천연가스 하이드레이트는 미래 청정 에너지원으로 주목받고 있다. 우리나라의 경우에도 동해안 울릉분지 부근 심해저에 천연가스 하이드레이트의 부존 가능성을 확인하였다. 이러한 심해저 천연가스 하이드레이트의 개발/생산을 위해서는 천연가스 하이드레이트의 상평형, 생성/해리 속도 등의 물리적 특성 뿐만 아니라 해리와 관련된 열물성에 대한 정보가 매우 중요하다. 따라서, 이 논문에서는 고압 마이크로 시차 주사 열량계를 이용하여 천연가스 하이드레이트의 주성분인 메탄, 에탄, 프로판 하이드레이트의 해리 엔탈피를 측정하였다. 메탄 하이드레이트의 경우 92.3 bar의 압력 조건에서 502.1 J/g-water, 437.3 J/g-hydrate, 54.2 kJ/mol-gas의 해리 엔탈피 값을 가졌으며, 해리 온도는 285.66 K이었다. 에탄 하이드레이트의 경우 18.7 bar에서 해리 엔탈피는 534.5 J/g-water, 438.8 J/g-hydrate, 73.8 kJ/mol-gas이었으며, 이때 해리 온도는 283.85 K이었다. 마지막으로 프로판 하이드레이트의 경우 압력이 3.2 bar일 때 해리 엔탈피는 417.2 J/g-water, 363.8 J/g-hydrate, 127.7 kJ/mol-gas 임을 알 수 있었고, 276.30 K에서 해리되는 것을 확인하였다. 또한, 해당 압력에서 각 기체의 가스 하이드레이트의 해리 엔탈피 측정과정에서 얻은 해리 온도를 문헌상의 가스 하이드레이트 3상(하이드레이트상(H)-물상(Lw)-기상(V)) 평형 데이터와 비교해 보면 거의 유사함을 알 수 있었다. 따라서, 고압 마이크로 시차 주사 열량계를 이용하여 가스 하이드레이트의 정확한 해리 엔탈피 측정 뿐만 아니라 가스 하이드레이트의 3상 평형도 함께 측정 가능함을 확인하였다. 【Gas hydrates are inclusion compounds formed when small-sized guest molecules are incorporated into the well defined cages made up of hydrogen bonded water molecules. Since large masses of natural gas hydrates exist in permafrost regions or beneath deep oceans, these naturally occurring gas hydrates in the earth containing mostly $CH_4$ are regarded as future energy resources. The heat of dissociation is one of the most important thermal properties in exploiting natural gas hydrates. The accurate and direct method to measure the dissociation enthalpies of gas hydrates is to use a calorimeter. In this study, the high pressure micro DSC (Differential Scanning Calorimeter) was used to measure the dissociation enthalpies of methane, ethane, and propane hydrates. The accuracy and repeatability of the data obtained from the DSC was confirmed by measuring the dissociation enthalpy of ice. The dissociation enthalpies of methane, ethane, and propane hydrates were found to be 54.2, 73.8, and 127.7 kJ/mol-gas, respectively. For each gas hydrate, at given pressures the dissociation temperatures which were obtained in the process of enthalpy measurement were compared with three-phase (hydrate (H) - liquid water (Lw) - vapor (V)) equilibrium data in the literature and found to be in good agreement with literature values.】

Natural methane hydrates occurring in marine clay sediments exhibit heterogeneous phase behavior with high complexity, particularly in the negatively charged interlayer region. To date, the real clay interlayer effect on natural methane hydrate formation and stability remains still much unanswered, even though a few computer simulation and model studies are reported. We first examined the chemical shift difference of 27Al, 29Si, and 23Na between dry clay and clay containing intercalated methane hydrates (MH) in the interlayer. We also measured the solid-state 13C MAS NMR spectra of MH in Na-montmorillonite (MMT) and Ca-montmorillonite (MMT) to reveal abnormal methane popularity established in the course of intercalation and further performed cryo-TEM and XRD analyses to identify the morphology and layered structure of the intercalated methane hydrate. The present findings strongly suggest that the real methane amount contained in natural MH deposits should be reevaluated under consideration of the compositional, structural, and physical characteristics of clay-rich sediments. Furthermore, the intercalated methane hydrate structure should be seriously considered for developing the in situ production technologies of the deep-ocean methane hydrate.

Large amounts of CH 4 in the form of solid hydrates are stored on continental margins and in permafrost regions. If these CH 4 hydrates could be converted into CO 2 hydrates, they would serve double duty as CH 4 sources and CO 2 storage sites. We explore here the swapping phenomenon occurring in structure I (sI) and structure II (sII) CH 4 hydrate deposits through spectroscopic analyses and its potential application to CO 2 sequestration at the preliminary phase. The present 85% CH 4 recovery rate in sI CH 4 hydrate achieved by the direct use of binary N 2 + CO 2 guests is surprising when compared with the rate of 64% for a pure CO 2 guest attained in the previous approach. The direct use of a mixture of N 2 + CO 2 eliminates the requirement of a CO 2 separation/purification process. In addition, the simultaneously occurring dual mechanism of CO 2 sequestration and CH 4 recovery is expected to provide the physicochemical background required for developing a promising large-scale approach with economic feasibility. In the case of sII CH 4 hydrates, we observe a spontaneous structure transition of sII to sI during the replacement and a cage-specific distribution of guest molecules. A significant change of the lattice dimension caused by structure transformation induces a relative number of small cage sites to reduce, resulting in the considerable increase of CH 4 recovery rate. The mutually interactive pattern of targeted guest–cage conjugates possesses important implications for the diverse hydrate-based inclusion phenomena as illustrated in the swapping process between CO 2 stream and complex CH 4 hydrate structure.

[1] Aperture distributions have been measured using replicas of apertures to investigate the effect of variable aperture on solute transport in five transparent fracture samples. The coefficient of variation (Cv), correlation length (λ), and anisotropy ratio (AR) were used to quantify the aperture distributions. Step-type tracer tests showed that dispersion is significantly influenced by both λ and Cv. Dispersivity (α) increases with λ and Cv. The magnitude of α is proportional to a polynomial function of natural logarithm of Cv2 and is linearly proportional to λ and the total sill measured from the semivariogram. The effect of AR is insignificant to those of the other parameters on α. Despite assumptions of the small perturbation theory developed by Gelhar [1993], the prediction of α from the theory is similar to α measured in this study. If the small perturbation theory is valid and reliable data for the tailing in a breakthrough curve are obtained, the analysis of the curve by the moment method gives a more reliable result than by the conventional fitting of a one-dimensional analytical solution to the curve.

[1] Three-phase equilibria (H-LW-V) for CO2 and NaCl solutions containing clay were measured to examine both electrolyte and capillary effect inside interlayer pores. Equilibrium line of 3 wt% NaCl and 10 wt% clay system was a little shifted to left side of 3 wt% NaCl solutions, which indicated that the injection of carbon dioxide into ocean sediments required higher pressure. CO2 hydrate formation kinetics was also attempted at 274.15 K and 30 bar. When clay minerals were added, the final consumption of CO2 decreased, but initial formation rate increased. Two-phase equilibria of CO2 hydrate and water solution eliminating gas phase particularly measured to simulate more closely actual deep sea condition. The overall results of this study would be used for estimating carbon dioxide injection depth and formation rate in sequestration process and making out solubility tendency in the deep sea.

The discharge coefficient of a long orifice in a rotating system is measured to examine the rotational effect on discharge behavior. The rotating system is comprised of a rotating disk and two stators on both sides of the rotating disk. Test rig is constructed to simulate the real turbine operating conditions. Pressure ratios between upstream and downstream cavities of the orifice range from 1.05 to 1.8, and rotational speed of the rotor disk is varied up to 10,000 rpm. The orifice hole bored through the rotor disk has length-to-diameter ratio of 10. For a better interpretation of discharge behavior, three-dimensional velocity field in the downstream and upstream cavities of the rotor is measured using a Laser Doppler Velocimetry. A new definition of the rotational discharge coefficient is introduced to consider the momentum transfer from the rotor to the orifice flow. Additional loss in the discharge coefficient due to pressure loss in the orifice hole at the inlet and exit regions is quantitatively presented in terms of the Rotation number and the compressibility factor. The effect of comer radiusing at the orifice inlet is also investigated at various rotational conditions.Copyright © 1998 by ASME