Effect of BOG nitrogen removal on PRICO liquefaction process

Basic load LNG plants mainly uses PRICO liquefaction process. For the factories which has nitrogen mole fraction over 1% in feed gas, the process of sending evaporation gas from storage tanks and loading stations back to the unit to recover methane, is likely to cause continuous accumulation of nitrogen in the unit, and the increase of evaporation volume and power consumption of vapor reliquefaction process. The modified PRICO process by removing nitrogen from the vapor after reliquefaction, and choosing proper nitrogen flashing pressure, in this way can reduce the vaporization rate of LNG product effectively, BOG output and the amount of refrigerant and the load of BOG compression engine and refrigerant compressor. HYSYS software is used to make simulation of the liquefaction and recovery unit of evaporation gas in one LNG factory and the load of compressor, load of cryogen and the amount of LNG under the conditions of denitrification as well as non-denitrification of evaporation gas are calculated respectively. The calculation results show that the amount of nitrogen in evaporation gas going through flash denitrification and reliquefaction in cold box is decreased from 34.62% before denitrification to 25.9% and the compressor power of evaporation gas using modified PRICO is decreased by 38.1%. The load of cryogen needed by the liquefaction of evaporation gas decreases by 46.5% and the amount of cryogen by 2.6%, and the load of corresponding cryogen compressor reduces by 284.3 kW and the amount of LNG by 7.2 t/d.