Analysis of the control factors for the evaporation rate of LNG storage tanks in LNG production enterprises

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In practice, for LNG production companies, it is essential to effectively control the evaporation rate of LNG storage tanks. On the one hand, it can increase the yield and boost production ； On the other hand, it can reduce energy consumption and eliminate unnecessary wasted effort. The daily evaporation rate for LNG storage tanks is specified as a design value during the design phase; for tanks with a capacity of 10,000 to 50,000 cubic meters, it is generally required that the daily evaporation rate, based on a full tank, be within 0.008%. Guided by this, the insulation and other supporting facilities are designed during the design phase, and after calculations (per the design specifications, the heat leakage is calculated for the tank bottom, top, side walls, connections, and anchor straps), the results are found to be satisfactory. Furthermore, the daily evaporation rate is also related to the type of tank; for example, due to its higher operating pressure compared to single-volume tanks, the daily evaporation rate of full-volume tanks is generally lower, usually within 0.005%. In terms of the loading method, the algorithms for frozen tanks, pressure tanks, and semi-frozen tanks are different. In terms of tank shape, there are differences between single containment, double containment, and full containment. Full containment is further divided into metal full containment and outer concrete type. The above refers to above-ground tanks; underground tanks are a different matter. The evaporation rate of LNG storage tanks, in terms of liquefaction production, can be divided into two parts: one is the throttling that occurs after exiting the cryogenic tank ； The other is tank evaporation. Based on the design size and type of the tank, general equipment suppliers can provide designed values for the daily evaporation rate of the tank. Generally, LNG storage tanks have a flash evaporation rate of 3–5‰ per day; this daily evaporation rate for LNG storage tanks is basically fixed and constitutes a key factor ； The content of natural gas components, such as non-condensable gases like nitrogen ； The export temperature of the finished LNG directly affects the amount of BOG generated upon the throttling of natural gas. It is basically calculated based on these three factors. In actual production, factors such as heat retention effects, the length of the finished product line, and the LNG tank level gauges also have an impact. The sources of evaporated gas are mainly as follows: First, liquid inlet flashing: the volume of gas displaced by the incoming liquid. Flashing gas in this context is taken into consideration only when there are fluctuations in the amount of LNG produced, fluctuations in the volume of gas entering the previous processing stage, or when the volume of liquid being processed is particularly large. When introducing new fluid, not only heat needs to be considered, but also the volume occupied by the incoming fluid. Because, at constant pressure, the volume of liquid entering is equal to the volume of gas being expelled. The amount of vaporization in this portion is calculated through simulation based on the specific components, pressure, and temperature of the LNG; it constitutes the main source of vapor from the LNG storage tanks, accounting for over 94% of the total. II. Self-evaporation of the storage tank: The amount of vapor generated in this process is calculated based on the evaporation rate of the tank, and it typically accounts for around 5% of the total amount. III. Return during loading: This portion of vaporized gas is the gas recovered through the gas-phase pipeline during the loading process, accounting for about 0.8–1% of the total amount. In light of the above situation, to effectively control the evaporation rate of LNG storage tanks, I believe the following aspects should be considered: First, energy-saving considerations should be taken into account in the design of LNG storage tanks. Essentially reduce the impact of factors such as the design aspects, as well as the material, volume, and shape of LNG storage tanks, on the evaporation rate. 1. It is recommended that spherical tanks be used as much as possible in the design of LNG storage tanks. Because, for the same volume, a sphere has the smallest surface area, and thus the smallest heat transfer area as well. Creating a vacuum using interlayers helps to achieve the best insulation effects. Additionally, since a spherical shape offers optimal pressure resistance, the design pressure of the storage tank can be increased, thereby reducing the amount of vapor that evaporates from the LNG storage tank. 2. For the selection of the loading pump, it is recommended to use a variable-frequency submersible pump. Since the loading pump is submerged in the liquid, it is not necessary to pre-cool the loading pump during LNG loading. In this way, when the variable-frequency submersible pump is not in use during loading, operating it at a reduced frequency for circulation can also prevent LNG from stratifying and rolling, thereby effectively reducing the pre-cooling required of the loading pump as well as the increase in vaporization gas during the loading process. Secondly, the loading time and quantity are not restricted. 3. The pipelines from the liquefied gas cryogenic tank to the storage tank, and from the storage tank to the loading arm, are made of stainless steel vacuum pipes. To reduce the increase in gasification of LNG during loading, caused by external temperature effects on the pipelines. Secondly, it is recommended to use the collected data for comparison in daily production operations, in order to continuously analyze and summarize operational experience. 1. Increase the cooling capacity of the liquefaction system to reduce the liquid outlet temperature of the liquefaction cryogenic tank. 2. Adopt the following liquid feeding method that focuses on liquid injection to prevent stratification and mixing within the LNG storage tank. 3. Optimize the cryogenic tank structure to separate non-condensable gases such as nitrogen within the tank for use as fuel or for venting. 4. Take advantage of the cooler periods of the day to concentrate on filling LNG. To reduce the high evaporation rate of LNG caused by pre-cooling, loading pumps, and other loading processes. 5. Using the flash vapor from the recovery tank for burning heat transfer oil can reduce gas consumption and, at the same time, lower the amount of non-condensable gases in the system.