End Flash Gas (EFG) compression
Expanding LNG market
Natural Gas (NG) from the well head contains impurities in different concentrations based on the source of the feedstock. Major impurities are nitrogen, water, acid gases (hydrogen sulfide, carbon & sulfur dioxide) and heavy hydrocarbons (ethane, butane, pentane and hexane) as well as condensates.
The “sour” gas first passes an acid gas removal system before the “sweet” gas is then compressed to around 42 to 45 bar and routed to the dehydration unit where water and some condensates are removed.
The dry, clean NG is then treated in a hydrocarbon processing unit where heavy hydrocarbons (NGL / natural gas liquids) are stripped from the gas stream in a propane chiller cycle.
The NG is then processed into Liquid Natural Gas (LNG) by chilling the gas step by step in a large heat exchanger – called main cryogenic heat exchanger (MCHE).
Typical EFG compressor, schematic
At the outlet of the MCHE (4) the LNG is still at high pressure (around 39 to 42 bar) and at a temperature of around -136 °C to -145 °C.
The pressurized LNG is flashed (expansion via Joule Thomson JT valve or expander) into a separator (or flash drum) (3) down to near atmospheric pressure (for storage) reaching -62 °C (saturation temperature of methane CH4). During this “flash” process, approximately 10% of the mass flow turns into gaseous flow. This vapor is called flash gas and contains, beside methane CH4, a large percentage of nitrogen N2 (the last impurity of the original).
The remaining (7) pure LNG (the liquid fraction contains above 98% methane CH4) is pumped via transfer pumps (5) to the LNG storage tanks.
As it is not economical to recycle the flash gas back into the liquefaction stream in order to increase CH4 extraction, the flash gas is used as fuel (6).
However, some of the valuable cold energy (gas at -162 °C) is extracted from the flash gas stream in a heat recovery exchanger (4). This extracted cold energy is then reused in the refrigeration loops (9) for increasing the LNG plant efficiency.
EFG handling
The “warm” (around -35 °C) EFG is compressed to around 25 to 30 bar and mixed with the BOG (Boil-Off Gas) stream routed from the LNG storage area. EFG and BOG are used as fuel (6) for the large mechanical drive gas turbines propelling the large refrigerant compressors. As the BOG flow undergoes large volume fluctuations between plant holding mode and LNG tanker loading mode, flexible compressors are of high importance.
EFG compression
The EFG compressor (8) units are medium-to-large, single-shaft, multi-stage centrifugal compressors of two-casing design with two or three intercooling stages. For highest process flexibility (large turndown at constant pressure ratio), the LP compressor casing is equipped with variable Inlet Guide Vanes (IGV) (10). The use of variable inlet guide vanes for this low-temperature application, pioneered by Siemens, results in a more economic solution, with very high reliability and availability.
Technology is the key
- 25 years of experience in large-scale low-temperature compressor application
- Dry gas seals (optional protected in heated / cooled seal carrier)
- Easy start-up and flow adjustment and high efficiencies over a wide operating range thanks to compressor control via adjustable IGV control
- For large-scale EFG compressors and weak electric grid, the IGV concept can be combined with start-up converters for the electric motor
Siemens solutions
Drives