High Capacity Concept for an Efficient Gas Expander FLNG Process

LNG plants with small-to-medium capacity rely frequently on refrigeration via work expansion of light gases. In many cases nitrogen is used as working fluid, but methane is also considered as eligible candidate. To achieve a fair efficiency gas expander processes are designed in such a way that two or more expanders are operated in different temperature ranges. Thus, the cooling duty for sub-cooling, liquefaction and in some cases, even pre-cooling of natural gas is provided by dedicated machines.

In a widely-used concept, which had been introduced first by Chris Dubar (e.g. expired US patent 5,768,912) two nitrogen expanders operate over the same pressure profile, but generate refrigeration at different temperatures, as the inlet temperature into the expanders is adjusted accordingly. A good thermodynamic process efficiency requires a narrow approach of heating and cooling curves. As the cooling duties are different in the respective cooling steps the expander operated at the cold end of the process will contribute only with a relatively small share to the overall refrigeration duty.

Obviously, the capacity of a single LNG train is governed by the maximum size of each piece of equipment. In a traditional dual gas expander process this bottleneck is established by the warm expander, which must provide 70-85% of the overall duty. An enforced duty balance between warm and cold nitrogen expander would cause an unacceptably high drop in process efficiency.

The objective of a joint development between Linde Engineering and Cryostar (both companies are members of The Linde Group) was to balance the duty of the warm and cold expander without sacrificing thermodynamic efficiency. As this goal cannot be reached with pure nitrogen as working fluid the well-known nitrogen-methane mixture was investigated. The full paper will disclose in detail the features of the (patent allowed) process, which is marketed under the name LIMEN^TM (LInde MEthane Nitrogen process).

After defining the operating conditions for an efficient gas expander process with equally loaded warm and cold expander, the corresponding equipment had to be selected. Cryostar is a leading-edge manufacturer of cryogenic machinery including large gas expanders. Cryostar identified a design for two large turbo-expanders with up to 16.5 MW shaft power. All relevant parameters (wheel diameter and material, specific speed, power density, tip speed, etc.) are covered by references and by adequate FEA calculations integrating the transmission torque, the centrifugal forces at trip speed and the axial thrust.

When two of these expanders (Cryostar size TC600/130) are integrated into the LIMEN^TM process, a double train LNG capacity of more than 2.5 mtpa LNG can then be achieved. This concept is a major step-forward for floating LNG projects, where economy-of-scale and lowest possible CAPEX are of the essence.