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Lim W.,Yonsei University | Choi K.,GS EandC | Moon I.,Yonsei University
Industrial and Engineering Chemistry Research | Year: 2013

Liquefied natural gas (LNG) is attracting great interest as a clean energy alternative to other fossil fuels, mainly due to its ease of transport and low carbon dioxide emissions, a primary factor in air pollution and global warming. It is expected that this trend in the use of LNG will lead to steady increases in demand over the next few decades. To meet the growing demand for LNG, natural gas liquefaction plants have been constructed across the globe. Furthermore, single train capacity has been increased to strengthen price competitiveness. To achieve greater capacity, more complex refrigeration cycle designs that combine two or more different conventional single refrigeration cycles are being developed to obtain synergistic effects in the liquefaction process. At the same time, a variety of recent studies have focused on designing suitable processes for offshore and small-scale plants to improve the profitability of stranded gas fields. LNG plants are known to be energy/cost-intensive, as they require a large amount of power for the processes of compression and refrigeration, and need special equipment such as cryogenic heat exchangers, compressors, and drivers. Therefore, one of the primary challenges in the LNG industry is to improve the efficiency of the current natural gas liquefaction processes in combination with cost savings. In this paper, we review recent developments in LNG processes, with an emphasis on commercially available refrigeration cycles. We also discuss recent research and suggest future directions for natural gas liquefaction processes. Up to this point, most studies have focused on operating cost. To achieve better results, future studies that investigate optimal design and operation of LNG technologies should consider both capital cost and operating cost. © 2012 American Chemical Society.


Kim S.,Yonsei University | Ko D.,GS EandC | Moon I.,Yonsei University
Chemical Engineering Transactions | Year: 2015

Pressure Swing Adsorption (PSA) is widely used process for gas separation. Recently, some researchers have been trying to use this PSA process for upgrading bio-gas. The bio-gas mainly consists of methane and carbon dioxide. Highly purified methane gas can be used for energy production, when the methane gas is separated from the carbon dioxide. However, during bio-gas extraction the composition and flow rate are slowly changing. Due to these changes, undesirable product gas properties of will be obtained. The efficiency of PSA process including recovery, purity and productivity are affected by operating conditions, such as feed pressure, feed velocity, p/f ratio, step-time etc. The aim of this research is dynamic optimization of PSA operation for bio-gas upgrading process considering feed composition variations. The objective is maximization of methane recovery, at the purity constraints, while control variables are step times for each step and Purge/Feed (P/F) ratio at regeneration step. In this research, robust PSA model is developed for dynamic simulation and optimization using gPROMS™ to solve problem. For improving accuracy of the model, distribution method is used; Central Finite Difference Method (CFDM), 2 level in this model. Especially, the time variables are treated as control variables in this model. Due to the discrete changes of boundary and equations, the solving of this optimization problem needs high skills and strategies. The 'SRQPD' solver, one of the NLP solvers, has been used, applying new equations with binary variables which can describe which time belongs to which step. Copyright © 2015, AIDIC Servizi S.r.l.,.


Lee I.,Yonsei University | Tak K.,Yonsei University | Kwon H.,Yonsei University | Kim J.,Yonsei University | And 2 more authors.
Industrial and Engineering Chemistry Research | Year: 2014

Natural gas liquefaction is an energy-intensive process in which energy reduction is a main concern. This research focused on minimizing the energy of the pure refrigeration cycle in natural gas liquefaction by improving the subcooling system. To minimize energy consumption, a pure refrigeration cycle with a subcooling system was simulated, and the result was thermodynamically analyzed. The thermodynamic analysis identified an opportunity to reduce the energy consumption, and a new design was proposed for the subcooling system. In addition, the proposed design was deterministically optimized to find the optimal compressing ratio, temperature, pressure, and flow rate. As the result, the optimal operating conditions were determined, and the energy consumption was reduced by 17.74%. © 2014 American Chemical Society.


Lim W.,Yonsei University | Lee I.,Yonsei University | Tak K.,Yonsei University | Cho J.H.,Yonsei University | And 2 more authors.
Industrial and Engineering Chemistry Research | Year: 2014

One of the most important challenges in a natural gas liquefaction plants is to improve the plant energy efficiency. In particular, if part of the natural gas is used as a fuel gas or the liquefaction ratio is taken into account as a design factor in an liquified natural gas (LNG) plant, process design focusing on cold energy recovery is an attractive option. In this study, various energy recovery-oriented process configurations and the potential improvements of energy savings in LNG plants were analyzed. Our primary focus for energy recovery in the LNG liquefaction process was centered on utilizing the flash gas stream from the phase separator. The applicability of the proposed configurations was validated by modeling and simulation of the single mixed refrigerant (SMR), propane precooled mixed refrigerant (C3MR), and single nitrogen (N2) expander processes. The simulation results for all cases exhibited considerable reductions of refrigerant flow rates, seawater cooling duties, and the specific work. For example, when the liquefaction ratio was fixed at 0.90, the amount of refrigerant was reduced by 4-5% by employing configuration 1, which recovers cold energy from the flash gas in LNG heat exchangers. This also led to 4-5% reductions of the specific work and seawater duty. Any energy recovery configuration will result in a considerable energy consumption reduction as the natural gas liquefaction process consumes a large amount of energy. Therefore, the optimization of energy recovery configurations in the natural gas liquefaction process is highly recommended with the objective of maximized energy savings considering capital costs. © 2014 American Chemical Society.


Tak K.,Yonsei University | Lee I.,Yonsei University | Kwon H.,Yonsei University | Kim J.,Korea Institute of Industrial Technology | And 2 more authors.
Industrial and Engineering Chemistry Research | Year: 2015

This study investigates the effects of multistage compression on single mixed refrigerant processes in terms of specific work. Comparison of specific work published in the literature is not straightforward due to the variety of compression configurations and the design bases. Therefore, four configurations (two-, three-, and four-stage and pump-added three-stage compressions) along with three natural gas compositions were considered. To compare with the simulation and optimization results in the literature, these 12 cases, having the same design basis, were optimized by adjusting the optimization variables such as the flow rate and composition of the refrigerant, the compression ratio of each compressor, the inlet pressure of the first compressor, and the outlet temperatures of the hot and cold refrigerant streams. There were two important findings: (1) adding a pump reduces specific work more than adding a compressor or decreasing the minimum temperature difference value in the compressors; (2) among the four configurations, the refrigerant composition does not significantly change, although it greatly affects the efficiency. The former results from the compressor constraint of the gaseous inlet and the latter from the minimum temperature constraint of the multistream heat exchanger. Furthermore, direct comparisons to other studies were also performed showing the importance of optimization and the effect of the design basis. © 2015 American Chemical Society.


Lee I.,Yonsei University | Tak K.,Yonsei University | Lim W.,Yonsei University | Moon I.,Yonsei University | Choi K.-H.,GS EandC
International Conference on Control, Automation and Systems | Year: 2012

The natural gas liquefaction process is an cryogenic system so it is very energy and cost intensive. There are several ways to reduce energy and cost consumption. One is getting optimal design and the other is getting optimal operating condition. After determined the process design, then optimize operating conditions such as temperatures, pressures and compositions. In this paper, to get optimal design in point of cost, optimization of driver selection is performed considering risk factor on C3-MR (propane pre-cooling mixed refregerant) process. Driver selection includes capital and operating cost. Capital cost could just get from venders but operating cost is very changeable. To solve driver selection more accurate, our team consider not only cost of fuels and electrics but also risk factor as the cost term. There are three scenarios to compare effects of risk factor. Risk factor depends on number of compressors which is connected to same driver. Each case is selected from mixed integer non-linear programming (MINLP) optimization results. The result is significantly different considering risk factor or not. © 2012 ICROS.

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