Natural gas liquefaction system and method with bi-circulating mixed refrigerant

Abstract

The invention discloses a natural gas liquefaction system and method with a bi-circulating mixed refrigerant. The system comprises a natural gas transporting pipe, a cool box, a heavy hydrocarbon separating device, a denitrifying device, an LNG(Liquefied Natural Gas) transporting pump, an LNG storage tank, a first cool circulating device and a second cool circulating device, wherein the cool amount generated by the first cool circulating device is used for cooling the second cool circulating device and raw natural gas, and the cool amount generated by the second cool circulating device is used for liquefying the raw natural gas. By the invention, according to the composition, pressure and temperature of the raw natural gas and the variation of ambient temperature of a project construction site, the composition and proportion of the mixed refrigerant can be reasonably configured, thereby acquiring different cooling and liquefaction temperatures to minimize the energy consumption of the whole process system. Meanwhile, in the practical operating process, because the raw natural gas is commonly from the underground gas field, the composition, pressure and other conditions can change, so that the cooling and liquefying temperature needs to be reasonably adjusted to maintain the operating efficiency and energy consumption of the natural gas liquefaction device.

Description

technical field [0001] The invention relates to the field of liquefied natural gas, in particular to a double-cycle mixed refrigerant natural gas liquefaction system and method. Background technique [0002] Natural gas liquefaction adopts the combined refrigeration cycle of propane precooling and mixed refrigerant refrigeration to cool and liquefy natural gas. The disadvantage is that the minimum precooling temperature of propane is about -40°C, and it is a fixed value, which cannot change with the temperature of the external environment. Therefore, when the composition of raw natural gas changes and the external ambient temperature changes with seasonal changes, it will cause the LNG output of the natural gas liquefaction plant to be unstable and the energy consumption of the plant operation to be high. [0003] figure 1 It is a schematic diagram of a natural gas liquefaction system in the prior art; figure 1 The meanings of the codes are as follows: 1-4 are propane pre-...

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Problems solved by technology

[0004] In the propane pre-cooling cycle, since the refrigerant is a single refrigerant, its pre-cooling temperature is a fixed value at a certain pressure, and the raw natural gas is a mixture, and its cooling process is a curve. Large heat temperature difference leads to lower heat transfer efficiency and increased overall energy consumption. In order to reduce the heat transfer temperature difference, a three-stage or four-stage propane decompression section is usually used to provide three to four pre-cooling temperature levels, thereby minimizing The heat transfer temperature difference of each temperature level is small, but in this way, three to four heat exchangers at different pressures, three to four propane compressor intake configurations at different pressures are required, and various supporting components are required. A series of facilities such as separators, pipelines, control systems, instruments, etc. greatly increase the complexity of the system, resulting in the disadvantages of natural gas liquefaction equipment, such as many equipment, large area, large initial investment, and complicated operation.

[0005] In addition, the geographical location of the project construction site is different, and the ambient temperature changes greatly, especially in some very cold areas, where the ambient temperature in winter reaches below -40°C. Cold temperatures will not require a propane pre-cooling cycle, but shutting down the propane pre-cooling will cause the entire unit to fail to operate, resulting in the inability to produce LNG products

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