BOG脱氮对PRICO液化工艺的影响

国内基本负荷型LNG工厂大多采用PRICO液化工艺,对于原料气中氮气摩尔分数超过1％的LNG工厂,储罐和装车站产生的蒸发气重新进入装置回收甲烷的过程,易导致系统内氮气含量不断累积,造成蒸发气量和蒸发气再液化过程动力消耗的增加.PRICO改进工艺通过对再液化后的蒸发气进行脱氮处理,合理选择氮气闪蒸压力,可以有效降低LNG产品的气化分率,减少BOG产生量,降低冷剂用量和BOG压缩机、冷剂压缩机的负荷.采用HYSYS软件对某LNG工厂的液化和蒸发气回收单元进行工艺模拟,分别计算了蒸发气脱氮气和蒸发气不脱氮气2种工况下的蒸发气压缩机负荷、冷剂负荷及LNG产量.计算结果表明：闪蒸脱氮后进入冷箱再液化的蒸发气氮气摩尔分数由脱氮前的34.62％降低至25.9％,改进型PRICO工艺蒸发气压缩机功率降低38.1％,液化蒸发气所需的冷剂负荷减少46.5％,冷剂量减少2.6％,相应冷剂压缩机的负荷降低328 4.3 kW,LNG产量减少7.2 t/d.（表1,图4,参8）

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. （1 Table, 4 Figures, 8 References）