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Research Papers: Energy Systems Analysis

Exergy Analysis for Air Separation Process Under Off-Design Conditions

[+] Author and Article Information
Li Yao

School of Metallurgical and
Ecological Engineering,
University of Science and Technology Beijing,
Beijing 100083, China;
Tangshan Tangsteel Gas Co. Ltd.,
Tangshan 063016, China

Lige Tong

School of Mechanical Engineering,
University of Science and Technology Beijing,
Beijing 100083, China;
Beijing Engineering Research Center for
Energy Saving and Environmental Protection,
University of Science and Technology Beijing,
Beijing 100083, China
e-mail: ligetongcn@163.com

Aijing Zhang, Yunfei Xie, Jianbiao Shen, Huazhi Li

School of Mechanical Engineering,
University of Science and Technology Beijing,
Beijing 100083, China

Li Wang

School of Mechanical Engineering,
University of Science and Technology Beijing,
Beijing 100083, China;
Beijing Engineering Research Center for
Energy Saving and Environmental Protection,
University of Science and Technology Beijing,
Beijing 100083, China

Shiqi Li

School of Metallurgical and
Ecological Engineering,
University of Science and Technology Beijing,
Beijing 100083, China

1Corresponding author.

Contributed by the Advanced Energy Systems Division of ASME for publication in the JOURNAL OF ENERGY RESOURCES TECHNOLOGY. Manuscript received February 27, 2014; final manuscript received February 15, 2015; published online April 8, 2015. Assoc. Editor: Abel Hernandez-Guerrero.

J. Energy Resour. Technol 137(4), 042003 (Jul 01, 2015) (5 pages) Paper No: JERT-14-1062; doi: 10.1115/1.4029911 History: Received February 27, 2014; Revised February 15, 2015; Online April 08, 2015

An air separation unit (ASU) represents the largest overall energy consumption (about 15–20%) in a steel production facility. Therefore, improving the operating efficiency of an ASU is an effective way to achieve energy savings and emission reductions. The exergy calculation program for an air separation process is developed, and the detailed exergy calculations and analysis for an actual ASU with capacity of 40,000 Nm3/h in service at Tangshan Tangsteel Gas Co. Ltd. are performed. The results show that the molar exergy contained in oxygen is the largest among all gaseous products, liquid argon contains the largest molar exergy among all liquid products, and liquid products of the same type have larger exergy values than their gaseous equivalents. In a same condition scenario (same environmental condition, same air feed mass flow at rated load operation of the expander), increasing liquids production is an effective way to enhance the process efficiency, especially by increasing liquid argon production at the rated load operation of the expander. The object efficiency of the process from the cleaning unit to production in an actual 40,000 m3/h ASU is 46.84%, while the simple efficiency of the cold box of the ASU is 64.31%. The largest amount of exergy loss is caused by the air compressor (AC), the packed-type air cooling tower (PACT), and the molecular sieve (MS) purifier. The cryogenic ASU itself is well operated from an exergetic viewpoint. On the basis of exergy analysis conducted, this study provides a reference for the improvement of the ASU analyzed and provides a reference for ASUs in general.

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Grahic Jump Location
Fig. 1

Diagram of the typical externally compressed cryogenic air separation process

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