(56b) Completing the Liquefaction Train by Using Two-Phase LNG Expanders

The primary method for liquefying natural gas is a cyclical process comprised of three basic steps. The process first compresses, then cools, and then expands the gas to lower the temperature. These steps are repeated until the gas liquefies. In the past, the expansion step was performed by a Joule-Thomson (JT) valve which both expands and cools the liquefied gas. Advancement in the process came about with the incorporation of a gas expander in the expansion step; however, even with a gas expander, a JT valve is still necessary to expand the liquefied gas to avoid vapor formation in the expander. The next step towards improving the liquefaction process is to combine the gas expansion with the liquid expansion in one machine, a two-phase expander. This removes the necessity of a JT valve and recovers the majority of the available energy from the two-phase LNG stream while further cooling the liquid portion by evaporation. This paper presents the design and performance of two-phase expanders.