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Closed loop self-cooling recuperative N2-expander cycle for energy efficient and ecological natural gas liquefaction process Muhammad Abdul Qyyum, Kinza Qadeer, and Moonyong Lee ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b04679 • Publication Date (Web): 28 Feb 2018 Downloaded from http://pubs.acs.org on March 2, 2018
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ACS Sustainable Chemistry & Engineering
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Closed loop self-cooling recuperative N2-expander cycle for energy efficient
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and ecological natural gas liquefaction process
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Muhammad Abdul Qyyuma, Kinza Qadeera, and Moonyong Leea, * a
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School of Chemical Engineering, Yeungnam University, Gyeongsan 712-749, Rep. of Korea
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Authors mailing address:
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1st author: Muhammad Abdul Qyyum
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Email ID:
[email protected] 9
Process System Design and Control Laboratory # 401, School of Chemical Engineering, Yeungnam
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University, 280, Daehakro, Gyeongsan, GyeongBuk, Republic of Korea.
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2nd author: Kinza Qadeer
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Email ID:
[email protected] 13
Process System Design and Control Laboratory # 401, School of Chemical Engineering, Yeungnam
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University, 280, Daehakro, Gyeongsan, GyeongBuk, Republic of Korea.
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Corresponding author: Moonyong Lee
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Email ID:
[email protected] 17
Process System Design and Control Laboratory # 401, School of Chemical Engineering, Yeungnam
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University, 280, Daehakro, Gyeongsan, GyeongBuk, Republic of Korea.
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*
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Prof. Moonyong Lee
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Email:
[email protected] 24
Telephone: +82-53-810-2512
Correspondence concerning this article should be addressed to:
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Abstract
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Liquefied natural gas (LNG) has attracted global attention as a more environmentally friendly energy
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source when compared to other fossil fuels. The nitrogen (N2) expander liquefaction is the most green
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and safe process among the different types of commercial natural gas liquefaction processes, but its
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relatively low energy efficiency is a major issue. In this study, an innovative closed loop self-cooling
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recuperation technology was proposed to reduce the exergy losses of the N2 expander LNG process.
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The LNG process with the implementation of the proposed technology was modeled using a
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commercial process simulation tool, ASPEN HYSYS® v9. Subsequently, a modified coordinate
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descent optimization algorithm was employed to achieve maximum potential benefits of the proposed
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technology. The energy efficiency of the proposed LNG process was further improved by energy
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recovery from end flash gas and high-pressure natural gas feed. Finally, the energy efficiency of the
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proposed closed loop self-cooling recuperative N2 expander LNG process was significantly improved
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up to 80.5% compared to the existing N2 expander based LNG processes, depending on the feed
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natural gas conditions, composition, and design parameters.
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Keywords: Natural gas liquefaction; LNG; Closed loop self-cooling recuperation; N2-expander
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liquefaction process; Ecological liquefaction process; End flash gas.
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ACS Sustainable Chemistry & Engineering
Introduction
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Compared to other fuels, liquefied natural gas (LNG) mainly has low carbon dioxide emissions
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and can meet the ever increasing stringent environmental safety rules and regulations. The location of
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the natural gas reserves is of key importance in the economics of natural gas storage and
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transportation. The main methods for storing and transporting natural gas is compression, liquefaction,
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and physical adsorption. Liquefaction of natural gas, resulting in a material with 1/600th the original
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volume, has been considered to be the most cost effective transportation strategy, especially when
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compared to gaseous natural gas transportation. However, liquefaction is an energy intensive process,
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and the liquefaction of 1 kg of natural gas consumes 1,188 kJ of energy 1, which is equivalent to 30–
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35% of the total energy required for LNG production.2, 3 Although, the energy requirement for LNG
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production strongly depends on the plant site conditions4, 5 and involved liquefaction process such as
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SMR, DMR, C3MR, and cascaded.6
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LNG processes can be categorized by their use of two major technologies, mixed refrigerant
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based processes, and N2 expander based processes. To optimize these processes for large scale LNG
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production, a combination of different refrigeration cycles has been developed such as, single mixed
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refrigerant (SMR), cascade, propane precooled mixed refrigerant (C3MR), and dual mixed refrigerant
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(DMR) processes. Among these, the SMR and N2 expander processes offer more simplicity with the
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lowest amount of capital investment, making them the most feasible for offshore LNG production.3, 7-
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SMR process has inherent safety and environmental concerns. The presence of highly flammable
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hydrocarbon based refrigerants makes SMR less attractive due to its environmental hazards and safety
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concerns.
The SMR process is highly energy efficient when compared to N2 expander processes, however, the
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Table 1 summarizes the safety and environmental data for different refrigerants. The safety data
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shows that N2 as a refrigerant has zero occupational exposure limit (OEL), and a zero lower
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flammability limit (LFL). Furthermore, the American Society of Heating, Refrigerating, and Air-
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Conditioning Engineers (ASHRAE) has standardized 34 safety groups (2010a and 2010b), and
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categorized N2 refrigerant in the A1 category (i.e. no flame propagation), whereas, all other
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refrigerants are categorized as A3 (i.e. highly flammable). The environmental data shows zero global
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warming potential (GWP) for N2 refrigeration when compared to the ingredients of the mixed
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refrigerant. Table 1. Safety and environmental data for refrigerants.12
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Safety data Refrigerant
Environmental data
OEL
LFL
Std 34 safety
(PPMv)
(%)
group
Methane (CH4)
1000
4.8
A3
0.0
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Ethane (C2H6)
1000
3.1
A3
0.0
~20
Ethylene (C2H4)
200
3.1
A3
0.0
