LNG refrigerant balance adjustment

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Adjustment of the cooling balance in LNG plants with mixed refrigerant cycles? When changes occur in the flow rate, composition, and inlet conditions of the natural gas to be liquefied, the original cooling balance of the plant is disrupted, and it becomes necessary to adjust the amount of refrigerant gas circulating in the system. The criteria for determining cold balance mainly depend on the temperature difference at the hot side of the heat exchanger and the temperature of LNG leaving the plant; the reference values are a temperature difference of around 3°C at the hot side of the mixed refrigerant flowing in and out of the cryogenic tank, with the temperature of LNG leaving the plant ranging from -163°C to -165°C. A large temperature difference at the hot side, or a low temperature of the LNG leaving the plant, indicates that there is excess cooling capacity. Conversely, if the temperature difference at the hot side is small, or if the temperature of the LNG leaving the unit is high, it indicates that the cooling capacity is insufficient. (1) Influence of mixed refrigerant components: Based on actual operational experience, the composition of the mixed refrigerant has a significant impact on the cooling capacity; it is therefore necessary to strictly control the composition of the refrigerant to keep it as close as possible to the designed composition. (2) Influence of feed gas composition: Changes in the composition of the feed gas lead to changes in the cooling capacity requirement. It mainly manifests in the following two situations: ① When the methane content approaches its lower limit, the content of heavier hydrocarbons (C2+) increases; as a result, more heavy-component liquids are washed out in the heavy hydrocarbon scrubber, and less gas enters the cold box. ②As the methane content increases, the content of heavier hydrocarbons (C2+) decreases; as a result, less heavy liquid fraction is washed out in the heavy hydrocarbon scrubber, and more gas enters the cold box. Case ① results in lower cooling capacity requirements for the mixed refrigerant cooling circuit, that is, the volume of air circulating is reduced ； In the ②nd case, the cooling capacity requirement of the mixed refrigerant cooling circuit increases, that is, the volume of air circulating increases. (3) Influence of the raw material gas flow rate entering the cold box: The raw material gas flow rate entering the cold box can change due to the following factors: ① The pressure at the inlet of the unit; if this pressure deviates by 0.2 MPa.G, it will result in changes in the compressor’s exhaust volume – as the pressure increases, the exhaust volume increases as well ； Pressure decreases, and the volume of exhaust gas reduces. Ignoring the impact of subsequent system leaks, it can be assumed that the natural gas flow rate into the cold box will change in the same way as the compressor’s exhaust volume. ②The temperature of the gas entering the device, as well as the temperature of the feed gas at the compressor inlet, affects the density of the gas, which in turn influences the compressor’s discharge volume. As the temperature rises, the exhaust volume decreases; as the temperature drops, the exhaust volume increases. The flow rate of the feed gas entering the cold box will change in the same way as the compressor’s exhaust volume. ③Wear of the compressor: After being in use for a period of time, the compressor’s components undergo wear, which affects the machine’s exhaust volume. ④The regeneration gas flow rate in the molecular sieve drying system; as the regeneration gas flow rate increases, the flow rate of raw natural gas entering the cold box decreases ； The flow rate of the regenerated gas decreases, while the flow rate of natural gas fed into the cold box increases.