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[en] The CO_2 hydrate formation experiments with THF (tetrahydrofuran) are performed in a stirred semi-bath reactor. The experimental data on CO_2 hydrate formation are obtained at constant pressure and temperature with the low driving force conditions. The experimental temperature is above 279 K, which is high enough to prevent the formation of only THF hydrate. The Gibbs free energy difference by the pressure variation is chosen as the driving force. The experimental results confirm that the THF drastically reduces the required CO_2 hydrate formation pressure. A two-parameter kinetic model based on the Chen-Guo model is developed to predict the CO_2 hydrate formation rate and to correlate the experimental data. It is found that the experimental correlation based on the present estimation model fits well with the experimental results and can predict the CO_2 hydrate formation rate satisfyingly for cooling application. - Highlights: • CO_2 hydrate with THF can be used as a secondary refrigerant for cooling application. • An experimental correlation was developed to predict the CO_2 hydrate formation rate. • The present model should be applied only for the CO_2 hydrate formation with THF.
[en] Graphical abstract: Mechanisms of CO_2 absorption and regeneration enhancement by nanoabsorbents. - Highlights: • CO_2 absorption/regeneration performance enhancement by using nanoabsorbents is evaluated. • SiO_2/methanol nanoabsorbent is more appropriate than Al_2O_3/methanol nanoabsorbent. • The regeneration rate of SiO_2/methanol nano-absorbent is improved by 22%. • The surface effect is more dominant than the thermal effect of the nanoparticle. - Abstract: The reduction of in the emissions of CO_2, which is the representative greenhouse gas, is actively investigated worldwide because of its contribution to global warming. Energy generation processes involving the gasification of fossil fuels separate the constituent gases before combustion occurs, rendering the capture of CO_2 more attainable. Generally, CO_2 is captured through an absorption method by using a liquid absorbent in large scale gasification systems. According to Henry’s solubility law, the absorption and regeneration processes should be operated at low and high temperatures respectively, and these require high energy consumption. As a solution, nanoparticles are added to the absorbent (methanol) to reduce energy consumption required in the absorption and regeneration processes. In this study, the absorption/regeneration performance was evaluated through a lab-scale combined CO_2-absorption/regeneration system. The nanoparticles used are SiO_2 and Al_2O_3, which are added at a 0.01 vol% concentration. In the case of the Al_2O_3/methanol nanoabsorbent, the performance decreases as the number of cycle increases, whereas the performance is improved steadily in the case of the SiO_2/methanol nanoabsorbent. Thus, the SiO_2 nanoparticles are more suitable for the combined CO_2 absorption/regeneration process. Furthermore, the mass transfer enhancement mechanisms of the absorption/regeneration process according to the addition of nanoparticles are presented.
[en] Graphical abstract: (a) Optical profiling image of the surface of copper after regeneration process in nanoabsorbents, Al_2O_3 (45 nm, 0.01 vol%). (b) The number of regeneration sites by the nanoabsorbents. - Highlights: • CO_2 regeneration performance is enhanced by using Al_2O_3 nanoabsorbents. • CO_2 regeneration process on the heating surface is visualized in nanoabsorbents. • Surface modification by nanoabsorbents has a greater effect than the nanoparticle size. • The mechanism of surface effect is the most plausible to explain the regeneration performance enhancement. - Abstract: Due to the recent increase in the consumption of energy and the use of fossil fuels, global warming has become a serious issue. To address this problem, CO_2 gas, which is the major element of the greenhouse gases, should be captured, regenerated and converted to useful fuels. The Integrated Gasification Combined Cycle (IGCC) and cement process generate large amount of CO_2, which are controlled through pre-combustion capture. However, this method has a disadvantage because the system temperature should be decreased to −20 °C or lower. Therefore, the development of new absorbent is required to reduce the energy consumed for refrigeration. There is a study that improved the CO_2 absorption performance by adding Al_2O_3 nanoparticles to methanol. However, studies on the regeneration of CO_2 in nanofluid absorbents (nanoabsorbents) are insufficient. Therefore, in this study, the CO_2 regeneration performance in Al_2O_3 nanoabsorbents is evaluated. It is found that the regeneration performance of CO_2 is improved by 16% by using nanoabsorbents compared to methanol. Furthermore, the CO_2 regeneration characteristics of nanoabsorbents are analyzed by considering the detachment time of CO_2 bubbles from the surface, the cross-sectional area of CO_2 bubble, and the number of regeneration sites through the CO_2 regeneration and bubble visualization experiments. It is concluded that the mechanism of surface effect is the most plausible to explain the CO_2 regeneration performance enhancement by nanoabsorbents.
[en] The objectives of this paper are to examine the effect of nano-particles on the pool type absorption heat transfer enhancement and to find the optimal conditions to design a highly effective compact absorber for ammonia/water absorption system. The effect of AL2O3 nano-particles and Carbon NanoTube(CNT) on the absorption performance is studied experimentally. The experimental ranges of the key parameters are 20% of ammonia concentration, 0∼0.08 vol% (volume fraction) of CNT particles, and 0∼0.06 vol% of Al2O3 nano-particles. For the ammonia/water nanofluids, the heat transfer rate and absorption rate with 0.02 vol% Al2O3 nano-particles were found to be 29% and 18% higher than those without nano-particles respectively. It is recommended that the concentration of 0.02 vol% of Al2O3 nano-particles be the best candidate for ammonia/water absorption performance enhancement
[en] The binary nanofluids which mean binary mixture with nano-sized particles are tested to apply to the absorption system. The dispersion stability of nano-particles in nanofluids in one of the most important factors for the absorption applications. The dispersion stability and the thermal conductivity of binary nanofluid are evaluated by the particles size analysis / visualization and by the transient hot wire method, respectively. To stabilize the nano-particles in a electrolyte, polymer is used as a stabilizer. It was found that the polymeric stabilizer was a good in stabilizing particles in a electrolyte. Also the ammonia/water based nanofluids (NH3/H2O+nanoparticles) showed a higher thermal conductivity than that of the base fluid (NH3/H2O).
[en] The necessity of the load following operation in nuclear power plants seems to be suggested because of the increased portion in total electricity supplied by nuclear power. However, in the case of load following operation, there is a shortage in the aspect to utilize uranium resources because the fuel once loaded cannot be saved. As a solution, we had presented the concept of the nuclear waste heat transport method which is to connect a secondary system with an absorptive cycle. At that moment, we developed a simple simulation model using PEPSE to check the feasibility of the proposed concept. In this paper, we developed an elaborated simulation model using PEPSE (Performance Evaluation of Power System Efficiencies) which is for a secondary system and EES (Engineering Equation Solver) which is for an absorptive system, and attempted to determine the design parameters and ultimately optimize the entire system
[en] Numerical analysis was carried out to examine the heat transfer and pressure drop characteristics of plate heat exchangers for absorption application using computational Fluid Dynamics(CFD) technique. A commercial CFD software package, FLUENT was used to predict the characteristics of heat transfer, pressure drop and flow distribution within plate heat exchangers. In this paper, a welded plate heat exchanger with the plate of chevron embossing type was numerically analyzed by controlling mass flow rate, solution concentration, and inlet temperatures. The working fluid is H2O/LiBr solution with the LiBr concentration of 50∼60% in mass. The numerical simulation show reasonably good agreement with the experimental results. Also, the numerical results show that plate of the chevron shape gives better results than plate of the elliptical shape from the view points of heat transfer and pressure drop. These results provide a guideline to apply the welded PHE for the solution heat exchanger of absorption systems
[en] Nanofluids are produced by dispersing nanoparticles in basefluid. Given its superior thermo-physical properties, nanofluids are gaining increasing attention and are showing promising potential in various applications. Numerous studies have been conducted in the past decade to experimentally and theoretically investigate thermal conductivity. The experimental finding is briefly summarized in this study; however, we do not intend to present a systematic summary of the available references from the literature. The primary objective of this study is to review and summarize the most debated mechanisms for heat conduction in nanofluids, such as the effects of a nanolayer, the Brownian motion of nanoparticles and aggregation, as well as induced convection. Finally, at a low concentration of nanoparticles, nanoconvection is the leading contributor to thermal conductivity enhancement, whereas at a higher concentration, the natural thermal transport along the backbone would aggregate, and the effects of the nanolayer would become significant and become ineligible.
[en] Recently there have been growing concerns that anthropogenic carbon dioxide (CO2) emissions cause the global warming problem. Therefore, the cutting edge technologies for the reduction, separation and collection of the CO2 are very important to alleviate this problem. The best methods for reducing the CO2 emission are to increase the energy efficiency and to remove it from the power plant. The CO2 absorption from the syngas in the integrated gasification combined cycle (IGCC) might increase the energy efficiency of the power generation systems, which also contribute to mitigate the global warming. In this study, the suspensions of nanoparticles in methanol called the nanofluid are developed and estimated to apply it to absorb CO2 gas in the IGCC systems. The nanofluids are prepared by the ultrasonic treatment and show the good stability. It is found that the CO2 absorption rate by the nanofluid is enhanced up to ∼8.3% compared to the pure methanol
[en] Stable suspension of the nanoparticles in the base fluids is inevitable to have the nanofluids be operated properly. Here we report the theoretical model to find the critical size of aggregates in nanofluids for the first time. The concept of relaxation time r_τ is adopted, which reflects the probability of encountering the particles. The hydrodynamic diameter of the aggregates in nanofluids must be kept below the critical size to be stably suspended, which is in good agreement with the experimental results.