Abstract

This study, a process design was carried out to recover the enormous amount of LNG cold energy that is abandoned in the conventional regasification system installed in offshore plant facilities such as LNG-FSRU. The effect of temperature on the thermal properties of LNG imported into Korea for the process design is examined. The results obtained through this study are summarized as follows.
1. Density, enthalpy, viscosity, heat capacity, and thermal conductivity of LNG in different countries showed different values according to temperature change even under constant pressure conditions. It was found that methane as the main composition of LNG had the greatest influence. In addition, since the difference in the composition of LNG by country affects the point of time of vaporization and the rapid change of the volume ratio of LNG, it is necessary to understand the change of thermal property according to the composition of LNG imported into Korea. Understanding property changing characteristics is an indispensable element in the process design of the system.

2. When the turbine size, thermal efficiency and exergy efficiency are considered in the process design of the regasification system equipped with the cold energy recovery process, the organic refrigerant R290 is used as the cold energy recovery operating fluid in the case of the single-stage cold recovery process. In the case of the two-stage cold recovery process, R170 was the organic refrigerant in the first organic Rankine cycle and R290 was the highest efficiency in the second organic Rankine cycle.

3. As a result of analysis the exergy loss for the process design of the regasification system with single-stage cold recovery process, was reduced by about 43.6% compared with the conventional open regasification system. In addition, the analysis of the available energy for the process design of the regasification system with the two-stage cold recovery process shows that the available energy is improved by about 12% compared with the process design of the regasification system with the single-stage cold recovery process.

4. In the process design of regasification system with cold recovery process, higher net output of turbine is obtained and LNG with higher methane content is applied, and thermal efficiency is also increased. Therefore, it was found that LNG composition ratio is a factor to be considered when designing the process for recovery cold energy of LNG.

5. When the PaCER system is additionally designed for the regasification system the lower the pressure at the outlet side of the expansion turbine, the greater the methane content is increased by 0.052%. Also, the methane number(MN) increased by 17.7% as the methane content increased. The thermal efficiency and exergy efficiency of the single-stage PaCER regasification system are improved by 4.0% and 6.0%, respectively, compared to the single-stage cold energy recovery system. The thermal efficiency and exergy efficiency of the two-stage PaCER regasification system are 2.6%, and 4.8%, respectively. In addition, the two-stage PaCER system shows 1.4% higher thermal efficiency and 2.4% higher exergy efficiency than the single-stage PaCER system.

6. While LNG is flowing at the cryogenic temperature inside the piping of the regasification system with a novel designed LNG cold energy recovery process, the LNG absorbs heat from the heat medium of cold energy recovery process. It can cause change. In this way, the various LNG flow patterns that can occur in the cold energy recovery process regasification system can be predicted by using some conventional flow simulation model. In addition, the effect of the pipe diameter, saturation pressure and pipe inclinations on the boundary of the cryogenic LNG flow pattern when the LNG flows through the horizontal process piping can be seen. Especially, it is possible to identify the dangerous sections such as burn-out that may occur while LNG is conducting heat exchange with the cold and hot working fluid inside the regasification system, and the flow boundary section which can increase the removal efficiency when removing heavy hydrocarbons from HHR.

7. A comparative analysis of annual net revenue considering the interest rate, electricity price. temperature of seawater used as the heat source of regasification system with LNG-FSRU cold energy recovery system, the minimum operating time and the amount of vaporization could be calculated. In addition, we could compare annual total net income according to LNG-FSRU operation time and amount of vaporization of LNG capacity, and calculate the BEP(Break-Even Point). Therefore, it is found that the PaCER system is the most efficient and economical when the novel proposed PaCER
system is secured the cold recovery system for at least 2,151hours and 238mmSCFD per year.

The results of the regasification system with the cold recovery process obtained through this study are expected to provide useful basic knowledge to improve the performance and efficiency of existing regasification system as well as to design new offshore plant facilities related to LNG in the future. In particular, if the results of the LNG cold energy recovery process design obtained in this study are linked to the heating and cooling industry, it can be used not only in the global warming mitigation measures but also in the cooling and heating industry in Korea. However, in order to apply the results obtained from the theoretical analysis in the present study to actual industrial fields, it should be sufficiently verified through experimental studies.