Study On Light Hydrocarbon Recovery Technology In LNG Receiving Station

As the demand of natural gas consumption is growing rapidly in China, importing liquefied natural gas (LNG) becomes an important way to guarantee the natural gas supply. However, the components of LNG vary from one production place to another. And most of LNG imported by China comes from Austria and Qatar where C2+content is high. The high content of C2+can affect the hydrocarbon dew point and the heat value of LNG, which is going to bring potential safety hazards and environmental problems. So it is essential to study light hydrocarbon recovery schemes in LNG terminals for the sake of the natural gas quality. In addition, light hydrocarbon can bring extra benefits to LNG terminals. Hence, combining natural gas light hydrocarbon methods and the characteristics of LNG terminals, light hydrocarbon recovery schemes of LNG terminals are studied.First, the available cold energy analysis model of LNG was developed based on P-R equation. And the cold energy of LNG was studied from the aspects of temperature, pressure and methane content. The results show that LNG contains large cold energy that changes along with temperature, pressure and methane content. Based on this, the cold energy can be used in LNG light hydrocarbon recovery systems.After that, condensation recovery method was chosen in the design. According to the position of LNG light hydrocarbon recovery process, the schemes are divided into low pressure schemes (scheme1-5) and high pressure schemes (scheme6). The low pressure schemes include multistage heat transfer schemes (scheme1-3) and single stage heat transfer scheme (scheme4-5). Direct heat exchange (DHX) process is introduced in scheme3.Third, grey box model was used to develop the exergy analysis model of LNG light hydrocarbon systems. Besides, the exergy model of equipment is based on black box model. The exergy analysis model can show the transmission, transformation, utilization and loss of exergy.At last, with the help of Aspen HYSYS7.2, cases of Bohai LNG terminal and Qingdao LNG terminal were simulated. Also benefits model was developed based on energy costs and products value.The results of theoretical study and case analysis show that all of the six schemes can achieve the goal of recovering light hydrocarbon in LNG, of which ethane recovery ratios are above90%. According to ethane recovery ratio, cold energy utilization and benefit analysis, scheme3provides the best performance. The ethane recovery ratio of scheme3is near98%, and the exergy efficiency of cold energy utilization unit is about80%. Besides, benefit of scheme3is the highest of all the schemes. So it is recommended scheme3to recovery light hydrocarbon in LNG terminals. Optimization of the plate number of the rectification towers in scheme3has been done. And after optimization of Qingdao LNG terminal, the plate number of demethanizer is24while that of dethanizer is16.