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Thermodynamic performance of ammonia in liquefied natural gas precooling cycle

ABSTRACT
The selection of proper refrigerants for natural gas liquefaction processes play a key role in cycle's efficiency. Mixed refrigerants have been proven to improve cycle's exergy efficiency over single pure refrigerant. However, the future of some of these refrigerants with higher global warming potential index (GWPI) are unknown due to the continuous restriction being enforced by the energy and environmental agencies over the past few decades. This study examines the benefits and drawbacks of mixing ammonia, a refrigerant with zero GWPI and a high occupational safety characteristic, with lighter hydrocarbon refrigerants such as methane and ethane as a mixed refrigerant in a natural gas liquefaction's precooling cycle. Results showed, presence of ammonia in mixed refrigerant not only saved in capital cost due to the smaller footprint of plant and smaller cold box, it also lowers the plants precooling operation expense by reducing the required compression power needed for the precooling cycle up to 16.2%. The results of exergy analyses showed that by reducing the molar concentration of more pollutant refrigerant methane and replacing it with ammonia enhanced the cycle's efficiency by 4.3% and lowered the heat exchanger total exergy loss up to 47.9 [kW.kgLNG-1].
KEYWORDS
PAPER SUBMITTED: 2020-12-27
PAPER REVISED: 2021-01-16
PAPER ACCEPTED: 2021-01-19
PUBLISHED ONLINE: 2021-02-06
DOI REFERENCE: https://doi.org/10.2298/TSCI201227072S
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