Oral Defense Examination : Optimization of Natural Gas Liquefaction and Boil-off Gas Reliquefaction

Speaker Harsha Nagesh Rao (Supervisor: Prof IA Karimi)

Host Department of Chemical and Biomolecular Engineering

Date/Time 09 Mar - 09 Mar, 10.00AM

Venue E4-04-03, Faculty of Engineering, National University of Singapore


Natural gas (NG) is the fastest growing fossil energy resource with its trade expected to increase significantly in the foreseeable future. Although pipelines are the best for transporting NG over short distances, today’s diverse and long supply chains necessitate global NG transport over long distances. For this purpose, liquefied natural gas (LNG) is the most common and economical option that has overcome geopolitical constraints on the global gas supply. However, natural gas liquefaction is capital- and energy-intensive. Besides, as LNG is maintained at cryogenic temperatures (about –160°C) at atmospheric pressure during its transport and storage, heat leakage into it is unavoidable despite extensive insulation. This vaporizes a part of the LNG to form Boil-Off Gas (BOG). BOG management at a regasification terminal is crucial. Storing BOG is impractical and flaring it is increasingly becoming unacceptable. Hence, most regasification terminals employ a system to reliquefy BOG, which consumes significant power. Considering these, in this work, we address the two crucial problems in the LNG supply chain, viz. optimization of natural gas (NG) liquefaction and optimization of BOG reliquefaction at regasification terminals.

We begin with the optimization of natural gas liquefaction. The process design and operational optimization of the liquefaction process need accurate and reliable models for the multistream heat exchangers (MHEXs) used in them. Hence, we develop a simple non-linear programming (NLP) equation-based model that enables MHEX design and flowsheet optimization, and optimize NG liquefaction cycles. Furthermore, we propose a data-driven geometry-independent NLP model to predict the performance of an existing MHEX, and subsequently optimize the overall liquefaction process.

Then, we study the optimization of BOG reliquefaction at LNG regasification terminals. We present a fundamental thermodynamic analysis of the BOG reliquefaction process, and evolve several new energy-efficient reliquefaction schemes. We optimize the reliquefaction schemes under idealistic conditions to obtain lower bounds on the total power consumption for a given BOG rate. Next, we study optimal design and costing of reliquefaction process using a comprehensive superstructure that includes all process options in the evolved reliquefaction schemes. We develop custom simulation modules for the process units in our superstructure and sequence their simulation to minimize several explicit optimization constraints. Considering realistic design specifications and operational constraints, we optimize a case-study terminal for various BOG rates and conditions.

Overall, this work significantly advances 1) the models for MHEXs used in the optimization of the NG liquefaction, and 2) the understanding of the BOG reliquefaction process in LNG regasification terminals.