Liquefaction process producing subcooled LNG
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.
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.
2. 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 to produce a nitrogen-rich feed gas supply;
Liquefying the nitrogen-rich feed gas supply with a main heat exchanger (MHE) to form nitrogen-rich, pressurized condensed LNG;
Introducing the nitrogen-rich, pressurized condensed LNG to a single-phase liquid expander (X1) contained within an initial surrounding vessel under inlet temperature (T1), inlet pressure (P1) and mass flow (M1);
Setting a rotational speed of X1 to expand the LNG to an outlet pressure (P2);
Operating X1 to process the nitrogen-rich pressurized condensed LNG;
Setting a rotational speed of a two-phase liquid expander (X2) contained within a second surrounding vessel and operating in series with the single-phase expander to decrease boil-off gas and optimize a ratio between liquid LNG (LLNG) and vapor LNG (VLNG); and
Operating X2 to remove nitrogen from the LLNG by evaporation, thus subcooling the liquid LNG, whereby the process is optimized by either maximizing one of the following values:
V 1=( T 1− T 3)/( M 1− M 3);
V 2= M 3/ M 1;
V 3=( T 1− T 3) M 3/ M 1;
V 5=( T 1− T 3)×( M 3− M 4);
V 6=( T 1− T 3)× M 3−( T 1− T 4)× M 4;
V 7=( T 1− T 3)× M 3/(( T 1− T 4)× M 4);
or minimizing the following value V4:
V 4= M 1− M 3;
with temperature T1 at the inlet to X1, temperature T3 of the liquid outlet of a phase separator (PHS), temperature T4 of the vapor leaving the PHS, mass flow M1 into X1, liquid mass flow M3 out of the PHS, vapor mass flow M4 out of the PHS and pressure P3 at a liquid LNG outlet, thereby reducing boil-off losses while producing LLNG at temperature T3.