Jonggeol Na

Effects of cation distribution on superexchange interaction in cobalt ferrites

The effects of the cation distribution change on the superexchange parameters and magnetic properties of cobalt ferrite powders were investigated. The cation distribution change of cobalt ferrite powders with quenching treatment was confirmed from the Mossbauer experiments. The exchange interaction parameters, i.e. J/sub AA/, J/sub BB/ and J(AB), and Neel temperature of slow-cooled and quenched CoFe/sub 2/O/sub 4/ powders were calculated from the variation of the magnetization with temperature on the basis of the two-sublattice model. With a quenching treatment of CoFe/sub 2/O/sub 4/, the thermal magnetization was changed, and this was attributed to the cation distribution change of cobalt ferrites. The absolute values of J/sub AB/ increased with the quenching treatment of CoFe/sub 2/O/sub 4/, while those of J/sub AA/ and T/sub N/ decreased. The values of J/sub BB/ of cobalt ferrite remained the same with the quenching treatment. This result suggested that the decrease of T/sub N/ of CoFe/sub 2/O/sub 4/ with quenching may be attributed to the decrease of J/sub AA/.

Simultaneous synthesis of a heat exchanger network with multiple utilities using utility substages

Abstract Heat exchanger network synthesis (HENS) has progressed by using mathematical programming-based simultaneous methodology. Although various considerations such as non-isothermal mixing and bypass streams are applied to consider real world alternatives in modeling phase, many challenges are faced because of its properties within non-convex mixed-integer nonlinear programming (MINLP). We propose a modified superstructure, which contains a utility substage for use in considering multiple utilities in a simultaneous MINLP model. To improve model size and convergence, fixed utility locations according to temperature and series connections between utilities are suggested. The numbers of constraints, discrete, and continuous variables show that overall model size decreases compared with previous research. Thus, it is possible to expand the feasible search area for reaching the nearest global solution. The models effectiveness and applications are exemplified by several literature problems, where it is used to deduce a network superior to that of any other reported methodology.

Influence Of Heat-treatment Conditions On the Eletrical Conduction Mechanism Of Grain And Grain Boundary In Cobalt Ferrites

The electrical resistance and conduction mechanism of the grain-boundary in Co/sub x/Fe/sub 3-x/O/sub 4/ (x=0.94, 0.97, 1.00, 1.03 and 1.06) under various heat-treatment conditions have been characterized, using a complex impedance analysis technique. The electrical resistance of cobalt ferrites decreases with increasing quenching temperature, which is mainly attributed to the decrease of R/sub gb/ in Fe-excess cobalt ferrites and R/sub g/ in Co-excess ones. It is verified that for the furnace-cooled Co/sub 0.94/Fe/sub 2.06/O/sub 4/, the conduction mechanism of the grain and grain-boundary is n-type and p-type, respectively. For furnace-cooled Co/sub 1.06/Fe/sub 1.94/O/sub 4/, the conduction mechanism of the grain and grain-boundary is p-type and n-type, respectively. These results indicate that the resistivity in the p-type part of cobalt ferrites decreases more rapidly than that in the n-type part with increasing quenching temperature. >

Microstructure and magnetic properties of high M/sub s/ ferrite/metal composite thin films

Metal/ferrite composite thin films were prepared by a reactive sputtering method to improve the saturation magnetization (M/sub s/). The M/sub s/ of the thin films increased with increasing the substrate temperature and this was attributed to the increase of the metal portion in the thin films. The Mossbauer result indicated that the ferrite phase in the thin films changed to hyperstoichiometric one with deposition of metal phase.

Optimal operation strategy of batch vacuum distillation for sulfuric acid recycling process

Abstract Vacuum distillation techniques are widely used in food, biological, pharmaceutical, and wastewater treatment industries. Because of its operation at low temperatures, vacuum distillation prevents the thermal decomposition of materials and alleviates corrosion processes; however, condenser size can be dramatically increased because of reductions in mean temperature differences under the vacuum operation. In batch vacuum distillation processes, vapor generation rate and mean temperature differences are changed with time. In view of these characteristics of batch operation, this paper suggests a novel methodology to minimize the condenser size in batch vacuum distillation processes. The target process is a sulfuric acid recycling system in semiconductor manufacturing plants. In this paper, an equation-oriented dynamic model is established and optimization problem is formulated. By solving the nonlinear programming problem, the condenser size is dramatically reduced when operation time is fixed. In contrast, operation time is greatly shortened when the installed condenser surface area is fixed.

Simultaneous Optimization Models for Heat Exchanger Network Synthesis with Multiple Utilities: A New Strategy by Using Utility Sub-stage

Abstract In simultaneous method area of heat exchanger network synthesis (HENS), a mixed integer nonlinear programming (MINLP) model with stagewise superstructure is developed to minimize the total annualized cost (TAC). However, most of the preceding researches do not allow to consider the multiple utilities. Unlike previous superstructure, the utility sub-stage is located between the stages and multiple utilities are arranged in series. The heuristic, the optimal utility position should not be located in splitting stream, increase the searching area by expanding the number of stage and reduce the model size which is related to convergence of model. To verify the model, two examples are proposed and they show the effectiveness by deducing a network superior to any reported methodology.

Risk-based underground pipeline safety management considering corrosion effect

Due to the long term usage and irregular maintenance for corrosion checks, catastrophic accidents have been increasing in underground pipelines. In this study, a new safety management methodology of underground pipeline, risk-based pipeline management, is introduced reflecting corrosion effect. First, principle of the risk-based pipeline management is presented compared with an original method, qualitative measure. It is distinguished from the qualitative measure by reflecting societal risk and corrosion in safety management of underground pipeline. And then, it is applied to an existing underground propylene pipeline in Ulsan Industrial Complex, South Korea. The consequence analysis is based on real information, and the frequency analysis reflects degree of corrosion. For calculation of corrosion rate, direct current voltage gradient (DCVG) and close interval potential survey (CIPS) are conducted. As a result of applying the risk-based pipeline management, risk integral is reduced by 56.8% compared to the qualitative measure. Finally, sensitivity analysis is conducted on variables, which affect the risk of the pipeline. This study would contribute to introduce quantitative measure to pipeline management and increase safety of pipeline.

A Comparative Study of Various Fuel for Newly Optimized Onboard Fuel Processor System under the Simple Heat Exchanger Network

− PEM fuel cell vehicles have been getting much attraction due to a sort of highly clean and effective transportation. The onboard fuel processor, however, is inevitably required to supply the hydrogen by conversion from some fuels since there are not enough available hydrogen stations nearby. A lot of studies have been focused on analyses of ATR reactor under the assumption of thermo-neutral condition and those of the optimized process for the minimization of energy consumption using thermal efficiency as an objective function, which doesn’t guarantee the maximum hydrogen production. In this study, the analysis of optimization for 100 kW PEMFC onboard fuel processor was conducted targeting various fuels such as gasoline, LPG, diesel using newly defined hydrogen efficiency and keeping simply synthesized heat exchanger network regardless of external utilities leading to compactness and integration. Optimal result of gasoline case shows 9.43% reduction compared to previous study, which shows the newly defined objective function leads to better performance than thermal efficiency in terms of hydrogen production. The sensitivity analysis was also done for hydrogen efficiency, heat recovery of each heat exchanger, and the cost of each fuel. Finally, LPG was estimated as the most economical fuel in Korean market.

Robust design of ambient-air vaporizer based on time-series clustering

Abstract A methodology for the robust design of an ambient-air vaporizer under time-series weather conditions is proposed. Two techniques are used to extract representative features in the time-series data. (i) The major trend of a day is rapidly identified by the discrete wavelet transform (DWT), in which a high level of Haar function reflects the trend of a day and drastically reduces the data size. (ii) The k-means clustering method groups the similar features of a year, and the reconstructed time-series dataset extracted by the centroids of clusters represents the weather conditions of a year. The results of the multi-feature-based optimization were compared with non-wavelet based and multi-period optimization by simulation under a year of data. The design structure from the feature extraction shows 22.92% better performance than the original case and is 12 times more robust in different weather conditions than clustering with raw data.