Korean Chemical Engineering Research

The Korean Institute of Chemical Engineers (한국화학공학회)
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Design and Economic Analysis of Low Pressure Liquid Air Production Process using LNG cold energy

LNG 냉열을 활용한 저압 액화 공기 생산 공정 설계 및 경제성 평가

  • Mun, Haneul (School of Chemical and Biomolecular Engineering, Pusan National University) ;
  • Jung, Geonho (School of Chemical and Biomolecular Engineering, Pusan National University) ;
  • Lee, Inkyu (School of Chemical and Biomolecular Engineering, Pusan National University)
  • 문하늘 (부산대학교 화공생명공학부) ;
  • 정근호 (부산대학교 화공생명공학부) ;
  • 이인규 (부산대학교 화공생명공학부)
  • Received : 2021.01.05
  • Accepted : 2021.04.19
  • Published : 2021.08.01
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Abstract

This study focuses on the development of the liquid air production process that uses LNG (liquefied natural gas) cold energy which usually wasted during the regasification stage. The liquid air can be transported to the LNG exporter, and it can be utilized as the cold source to replace certain amount of refrigerant for the natural gas liquefaction. Therefore, the condition of the liquid air has to satisfy the available pressure of LNG storage tank. To satisfy pressure constraint of the membrane type LNG tank, proposed process is designed to produce liquid air at 1.3bar. In proposed process, the air is precooled by heat exchange with LNG and subcooled by nitrogen refrigeration cycle. When the amount of transported liquid air is as large as the capacity of the LNG carrier, it could be economical in terms of the transportation cost. In addition, larger liquid air can give more cold energy that can be used in natural gas liquefaction plant. To analyze the effect of the liquid air production amount, under the same LNG supply condition, the proposed process is simulated under 3 different air flow rate: 0.50 kg/s, 0.75 kg/s, 1.00 kg/s, correspond to Case1, Case2, and Case3, respectively. Each case was analyzed thermodynamically and economically. It shows a tendency that the more liquid air production, the more energy demanded per same mass of product as Case3 is 0.18kWh higher than Base case. In consequence the production cost per 1 kg liquid air in Case3 was $0.0172 higher. However, as liquid air production increases, the transportation cost per 1 kg liquid air has reduced by $0.0395. In terms of overall cost, Case 3 confirmed that liquid air can be produced and transported with $0.0223 less per kilogram than Base case.

본 연구에서는 액화천연가스(LNG; liquefied natural gas) 재기화 과정에서 버려지는 냉열을 회수하는 방법으로 액화 공기를 생산하는 공정을 개발하였다. 액화 공기는 LNG 수출국으로 운송하여 천연가스 액화를 위한 냉매를 부분적으로 대체하는 용도로 활용될 수 있다. 이를 위하여, 액화 공기는 LNG 운반선에 저장 가능한 압력을 만족하여야 한다. 따라서, 가장 널리 사용되는 멤브레인 탱크로 액화 공기를 운송하기 위해 약 1.3 bar에서 공기가 액체 상태로 존재할 수 있도록 설계하였다. 제안한 공정에서, 공기는 LNG와의 열교환 이후 추가적인 질소 냉매 사이클과의 열교환을 통해 과냉된다. LNG 운반선의 최대 용량만큼 액화 공기를 생산할 때 운송비용 측면에서 가장 경제적일 수 있으며, 천연가스 액화공정에서 활용할 수 있는 냉열이 많아지게 된다. 이를 비교하기 위하여, 동일한 1 kg/s의 LNG 공급 조건 하에서 기존 공정을 이용한 Base case와 제안공정 내 유입 공기 유량을 각각 0.50 kg/s, 0.75 kg/s, 1.00 kg/s으로 하는 Case1, Case2, Case3를 구성하고 열역학적 및 경제적 측면에서 분석하였다. 액화 공기 생산량이 많을수록 1kg의 생산량 당 더 많은 에너지가 요구되는 경향을 보였으며 Case3는 Base case 대비 0.18 kWh 높게 나타났다. 그 결과 Case3의 액화 공기 1 kg 당 생산 비용이 $0.0172 더 높게 나타났다. 그러나 액화 공기의 생산량이 증가함에 따라 1 kg 당 운송 비용이 $0.0395 감소하여 전체 비용 측면에서 Case3는 Base case에 비해 1 kg 당 $0.0223 적은 비용으로 액화 공기를 생산 및 운송할 수 있음을 확인하였다.

Keywords

Acknowledgement

본 연구는 부산대학교 기본연구지원사업(2년)에 의한 연구임.

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