Boil off gas CYCLE
The BOG cycle is an independent loop that consists of the pre-cooler, BOG-compressors, plate-fin cryogenic, separator, vent gas heater and LNG return pump. The plate fin cryogenic heat exchanger and the separator.
Boil-off gas is removed from the LNG tanks by means of a centrifugal BOG compressor. The BOG is compressed to 3.5 bar g and then cooled and condensed at this pressure.
To enable stable temperatures at inlet to the heat exchangers, a pre-cooler is installed upstream of the BOG compressor with a temperature controller. The pre-cooler is controlled based on both the BOG compressor suction temperature and discharge temperature.
The condensate (LBOG) leaves the cryogenic platefin heat exchanger at a temperature of about 159.5°C. Incondensable gases are separated in the separator and routed to the flare units where the entrained methane is burned. Liquid leaves the separator by level control and is returned to the storage tanks driven by the differential pressure between the separator pressure and storage tank pressure.
The LNG return pump is normally not used during normal operation. However, if reliquefaction is done by free flow, the differential pressure between the separator and storage tank is only the liquid column in the separator, which may not be enough to push the liquid to the tanks. During this operating mode the LBOG return pump are used to push the LBOG to the cargo tanks.
NITROGEN CYCLE
The refrigerant capacity is produced by a nitrogen compression/expansion cycle. Nitrogen gas at a pressure of 13 bar a is compressed to 56 bar a in a 3-stage centrifugal compressor. The nitrogen is cooled by fresh water after each stage of compression.
After the final compression and after cooling, the nitrogen is led to the “warm” section at the top of the cryogenic heat exchanger in which the nitrogen is pre-cooled to approximately –110°C. The nitrogen is then expanded from approximately 56 bar a to an outlet pressure of approximately 13 bar a and –163°C outlet temperature. The nitrogen is then routed to the “heat exchanging” section at the bottom of the cryogenic heat exchanger. The cold nitrogen then absorbs heat from the BOG and warm high-pressure nitrogen.
The nitrogen flows through from bottom of the cryogenic heat exchanger to top before it is returned to the suction side of the 3-stage compressor.
CAPACITY CONTROL
The system capacity is controlled as follows:
The BOG compressors are equipped with controle valves, which adjust the compressor flow rate to achieve the storage tank set pressure.
The N2 refrigeration system capacity control is done by changing the amount of N2 that is circulated through the N2 refrigeration loop. This is done by adding or removing N2 to the circuit, which will increase the pressure, density and therefore circulated mass flow.
Composition Mole %
Nitrogen 0.3242
Methane 93.1563
Ethane 6.2409
Propane 0.1758
i-butane 0.0001
n-butane 0.0065
i-pentane 0.0634
n-pentane 0.0321
Hexanes+ 0.0007
