Abstract

Offshore natural gas liquefaction has become increasingly economically viable as the value of LNG has increased during the past few years. This has increased interest in both onshore and offshore liquefaction projects. Offshore liquefaction may facilitate the introduction of new technologies and processes because their project economics favour different criteria. For offshore applications, process design criteria such as compactness, weight, modular design, and process safety become more important.

This paper considers the potential process improvements that could be realised for mixed refrigerant-based processes using two-phase expander. This type of expander has recently been developed for commercial application and operated on a small-scale LNG project in Poland. Integration of two-phase expanders into mixed refrigerant processes offers the potential to decrease equipment footprint, installed compressor capacity, and flammable liquid inventory but these expanders are still new and have yet to be proven in a large capacity plant. This paper shares the simulation results for dual and single mixed refrigerant processes using three different expansion devices including two-phase expanders. These results support a discussion of the potential benefit of improved expander operating range on process and facility design. For onshore liquefaction, expanders would likely be used to boost efficiency to increased LNG production for a defined compressor and driver. Offshore however, two-phase expanders could enable use of single cycle mixed refrigerant liquefaction processes. Such processes benefit greatly from the use of two-phase expansions and may offer the potential to decrease footprint, weight, complexity, and flammable fluid inventor. All advantages that are highly relevant to offshore liquefaction facilities. The results encourage more detailed analysis of processes incorporating two-phase expanders for offshore liquefaction.

Introduction

Increasing global demand for natural gas is supporting the rapid growth of worldwide LNG production capacity. As demand continues to grow and the value of natural gas remains high, the impetus to monetise non-traditional gas resources also grows. Offshore floating LNG production has generated interest because it offers the potential to:

  • avoid flaring or reinjection of associated gas

  • monetise smaller or remote fields of non-associated Gas

  • reduce exposure to public and increase security of Facilities

  • lower LNG production costs The realisation of large floating production, storage, and offloading (FPSO) facilities for oil production and LPG production, use of barge transport for the Snøhvit LNG facility, and other developments demonstrate the potential for offshore LNG.

Liquefaction Cycle Development (1, 2, 3, 4).

The base-load LNG industry now has over 40 years of history starting with permanent operations of the Camel plant in Algeria in 1964. The earliest plants consisted of fairly simple liquefaction processes based on either cascaded refrigeration or single mixed refrigerant (MR) processes with train capacities less than one million tonnes per annum (MTPA). These were quickly replaced by the two-cycle propane pre-cooled mixed refrigerant (C3MR) process developed by Air Products and Chemicals Inc. (APCI). This process became the dominant liquefaction process technology by the late 1970s and is stillcompetitive in many cases.

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