Mixed refrigerant and nitrogen expansion combinational refrigeration type natural gas liquefying method

Abstract

The invention belongs to the technical field of gas liquefaction and relates to a mixed refrigerant circulation and nitrogen expansion circulation combinational refrigeration type natural gas liquefying method. The method comprises the following steps of: allowing natural gas subjected to pretreatments such as acid gas removal, dehydration and the like to enter a natural gas liquefying section cooling box; performing refrigeration by vaporizing the mixed refrigerant in the liquefying section cooling box and controlling the pressure to be 4 to 10 Mpa so as to liquefy the natural gas under high pressure, wherein the temperature of the liquefied natural gas (LNG) departing from the liquefying section cooling box is between -75 and -110 DEG C; cooling the LNG to about -150 DEG C after the LNGenters a supercooling section cooling box; reducing pressure to be the pressure of a storage tank by using a liquid expander and further cooling to about -160 DEG C; and then compressing the LNG to serve as fuel gas. The method has the advantages of simple technical process, high reliability, high liquefaction rate and suitability for industrialized production.

Description

technical field [0001] The invention belongs to the technical field of gas liquefaction, and relates to a natural gas liquefaction process, in particular to a refrigeration type natural gas liquefaction method using a mixed refrigerant cycle and a nitrogen expansion cycle. Background technique [0002] At present, the processes used for natural gas liquefaction at home and abroad mainly include: nitrogen expansion refrigeration process, cascade refrigeration process, single mixed refrigerant process, mixed refrigeration process with propane pre-cooling, and double mixed refrigerant process. Each has its own advantages and disadvantages. The nitrogen expansion refrigeration process is simple, but the efficiency is low, and it can only be used in small-scale liquefaction devices such as peak-shaving type; the cascade refrigeration process has high efficiency, and since the three refrigeration cycles are all pure components, design calculation and operation comparison It is co...

AI Technical Summary

Problems solved by technology

The nitrogen expansion refrigeration process is simple, but the efficiency is low, and it can only be used in small-scale liquefaction devices such as peak-shaving type; the cascade refrigeration process has high efficiency, and since the three refrigeration cycles are all pure components, design calculation and operation comparison It is convenient, but the process equipment and compressor units are many, the purity requirements of the pure components of the refrigerant are high, the heat exchange equipment is many and complex, and the equipment investment is high; the single mixed refrigerant process is simple, and the efficiency is higher than that of the cascade refrigeration process Low, because there is only one refrigeration cycle, and the pressure of gaseous refrigerant after throttling is low, so the volume flow rate of gaseous refrigerant is large, and it is limited by compressors, cold boxes and other equipment when it is applied to medium to large devices, so it is generally only used in For small and medium-sized liquefaction plants, another disadvantage of this process is that due to the large operating temperature range of the refrigerant, a strict refrigerant component distribution ratio is required, otherwise the thermal efficiency will be greatly reduced, so it is necessary to purchase and store multiple pure refrigerants. components, configuration is difficult, and start-up is slow; both the mixed refrigerant process with precooling and the two-stage mixed refrigerant process have high thermal efficiency, and can be applied to large-scale natural gas liquefaction plants, but these two processes are relatively Complex, with a large number of equipment, and the refrigerant system has similar disadvantages to the single-mixed refrigerant system, that is, the refrigerant component ratio is relatively strict, configuration is difficult, slow start-up, outsourcing and storage of multiple pure components, etc., so Generally, the economic value is only reflected when it is applied to large-scale liquefied natural gas installations

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