Evaluation and optimization of BOG Re–condenser in the LNG reservoir process

Abstract

The liquefied natural gas (LNG) is a product of condensation of natural gas in very low temperature and stored at ambient pressure in the cryogenic tanks. For using, the LNG is re-gasified and supplied to power and chemical plants as fuel and material. Recently, the demand of using LNG has increased significantly because it is abundant, low price and CO2 emission. Therefore, the LNG plays an important role in the global energy system as an input to power generation, heating and material in industry. Many projects of LNG receiving terminal are being constructed in Vietnam and operated in the next few years. However, during storage in the cryogenic tank, a considerable amount of LNG evaporates by solar absorption and generates boil–off gas (BOG) which causes of loss increasing and unsafety. As a result, the BOG generation problem is highlighted for operation optimization in the future. In this study, a retrofit BOG re–condensation process was proposed based on the existing plant at LNG Dapeng receiving terminal, China. The previous study evaluated the process only on energy analysis, hence the irreversibility was not realized. As a result, the total lost, 1130.9 (kW), in term of exergy loss is really high. Therefore, the exergy analysis method was considered to optimize the operating parameters of the process. The results indicated that the exergy loss was decreased by 2.6 times compares to the previous process, it reduces to only 434.9 (kW). In addition, BOG was completely condensed and optimum operating pressure of the condenser is determined in the range of 8–9 bars. Thus, the results give the engineers more options to select the optimum conditions for reduction of energy consumption and environment savings.