Liquefaction process producing subcooled LNG

摘要

A variable speed liquid LNG expander (X1) and a variable speed two-phase LNG expander (X2) in line, downstream from X1. The rotational speed of both expanders can be controlled and changed independent from each other. The speed of expander X1 and expander X2 is determined in such way that the amount of liquid LNG downstream from the PHS compared to the feed gas supply is maximized and the amount of vapor and boil-off downstream of X2 is minimized.

主权项

1. A method for optimum sub-cooling of LNG to reduce boil-off losses at a liquefaction plant that produces LNG, the method comprising the following steps:
Providing a source of substantially nitrogen-free LNG; Injecting nitrogen into pressurized natural gas with a nitrogen injector at the liquefaction plant as a first step in sub-cooling the substantially nitrogen-free LNG, thereby producing a nitrogen-rich feed gas supply; Liquefying the nitrogen-rich feed gas supply with a main heat exchanger to form nitrogen-rich, pressurized condensed LNG; Introducing the nitrogen-rich pressurized condensed LNG to a single-phase liquid expander contained within an initial surrounding vessel under inlet temperature, inlet pressure and inlet mass flow; Setting a rotational speed of the single-phase expander to expand the LNG to outlet pressure; Operating the single-phase expander to process the nitrogen-rich pressurized condensed LNG; Setting a rotational speed of a two-phase liquid expander contained within a second surrounding vessel operating in series with the single-phase expander to decrease boil-off gas and optimize a ratio between liquid LNG and vapor LNG; and Operating the two-phase expander to process the nitrogen-rich pressurized condensed LNG received from the single-phase expander and separating the nitrogen from the LNG by evaporation of the nitrogen, whereby the process is optimized to produce subcooled liquid LNG to reduce boil-off losses.