Exergoeconomic Evaluation of an Integrated Nitrogen Rejection Unit with LNG and NGL Co-Production Processes Based on the MFC and Absorbtion Refrigeration Systems

Natural gas is often associated with nitrogen and heavy compounds. The Heavy components in the natural gas not only can feed downstream units, owing to the low temperature process may be formed solid as well. Therefore, heavy components separation can be a necessity and produce useful products. Virtually, all natural gases are containing nitrogen ​​that would lower the heating value of natural gas. Removing nitrogen from natural gas at a concentration of more than 4% can be vital. Integration of the natural gas liquids (NGL), liquefied natural gas (LNG) and nitrogen rejection unit processes is an effective procedure which can reduce the required refrigeration. In the present paper, a novel mixed fluid cascade natural gas liquefaction process is investigated by exergy and exergoeconomic analysis methods. one of the vapor compression cycles is replaced with a water-ammonia absorption refrigeration cycle. The results include cost of exergy destruction, exergoeconomic factor, exergy destruction and exergy efficiency. Results of exergoeconomic analysis indicates that the maximum exergoeconomic factor, which is 89.49%, is related to the HX8 in the water-ammonia absorption refrigeration cycle and the minimum exergoeconomic factor, which is 0.0026%, is related to the HX2 in the liquefaction cycle. In this process, the fourth compressor has the highest exergy destruction cost (5750307 $/hr) and HX8 in the absorption refrigeration cycle has the lowest exergy destruction cost (2.033 $/hr). Due to the high value of fuel cost rate in compressor, their exergy destruction cost is much higher than other devices.