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1. Classification of natural gas liquefaction process
At present, the types of natural gas liquefaction processes are mainly divided based on their functions and refrigeration methods.
(1) According to their functions, they can be divided into basic load liquefaction units and peak-shaving liquefaction units. Small-scale LNG units belong to peak-shaving liquefaction units.
(2) According to the refrigeration method, it can be divided into: ① cascade liquefaction process; ② mixed refrigerant liquefaction process, including closed, open, propane precooling, CII, etc.; ③ liquefaction process with expander, including natural gas expansion, Nitrogen expansion, nitrogen-methane expansion, etc.
However, the above division is not strict, and a composite process is usually adopted that includes different combinations of certain parts of the various liquefaction processes described above, and each method contains multiple types.
2. Types and composition of natural gas liquefaction plants
The types of natural gas liquefaction units mainly include base load liquefaction units, peak-shaving liquefaction units, floating LNG production storage and unloading units and LNG receiving terminals, and their definitions are as follows.
(1) Basic load liquefaction plant: refers to the large-scale liquefaction plant produced for local use or external transportation.
(2) Peak-shaving liquefaction device: refers to a natural gas liquefaction device for peak-shaving load or to supplement winter fuel supply, usually liquefying and storing excess natural gas during low-peak load and re-vaporizing for use in peak or emergency situations.
(3) Floating LNG production, storage and unloading device: It is a new type of natural gas liquefaction device in marginal gas fields and offshore gas fields. It is favored for its advantages of low investment, short construction period and easy demolition.
(4) LNG receiving terminal: refers to the device that receives the LNG transported by LNG carriers from the basic load natural gas liquefaction device, generally equipped with a liquefaction recovery system for LNG tank top boil-off gas BOG (Boil Off Gas).
Natural gas liquefaction plant is generally composed of natural gas pretreatment process, liquefaction process, storage system, control system and fire protection system, among which liquefaction process is the core part of natural gas liquefaction plant. Large-scale LNG plants generally include several sets of natural gas liquefaction plants, and each set of liquefaction plants may have multiple production lines. Due to the different production purposes of different liquefaction units, there are naturally large differences in their specific compositions.
3. LNG refrigeration method
The so-called refrigeration refers to the use of artificial methods to create low temperature (below ambient temperature) technology. Refrigeration methods mainly include the following three.
(1) Use the endothermic effect of material phase transitions (such as melting, evaporation, sublimation) to achieve refrigeration. The so-called vapor refrigeration refers to the use of liquid evaporation to achieve refrigeration. Vapor refrigeration can be divided into three types: vapor compression (mechanical compression), vapor injection and absorption. At present, vapor compression refrigeration is mostly used.
(2) Use the cooling effect of gas expansion to achieve refrigeration. Gas expansion refrigeration currently widely uses turbine expansion refrigeration, and also uses throttle valve refrigeration and heat separator refrigeration.
(3) Use the thermoelectric effect of semiconductors to achieve refrigeration.
In the natural gas liquefaction process, liquid evaporation and gas expansion are widely used to achieve refrigeration. Throttling refrigeration must have high enough pressure energy to be used, and the efficiency is low. It is generally used in situations where the pressure of the raw gas is high and the amount of liquefaction required is small.
4. Common natural gas liquefaction process
Different liquefaction processes have different refrigeration methods. In the natural gas liquefaction process, the common natural gas liquefaction process mainly includes cascade liquefaction process, mixed refrigerant liquefaction process and liquefaction process with expander, and their refrigeration methods are as follows.
(1) Cascade liquefaction process
It consists of several overlapping refrigeration cycles operating at different temperatures, in which the high, medium and low temperature parts use high, medium and low temperature refrigerants respectively. The evaporation of the refrigerant in the high temperature part is used to condense the refrigerant in the low temperature part, and the refrigerant in the low temperature part is re-evaporated to output the cooling capacity, and these parts are connected by several evaporative condensers. The evaporative condenser is both the evaporator of the high temperature part and the condenser of the low temperature part. for natural gas
For liquefaction, a three-stage cascade refrigeration cycle with propane, ethylene and methane as refrigerants is mostly used.
(2) Mixed refrigerant liquefaction process
The process was evolved from the cascade refrigeration process in the late 1960s. Hydrocarbon mixtures (N2, C1, C2, C3, C4, C5) are mostly used as refrigerants to replace multiple pure components in the cascade refrigeration process, and the composition is determined according to the composition and pressure of the feed gas. Taking advantage of the characteristics of the heavy components in the multi-component mixture condensing first and the light components condensing later, the cooling capacity of different temperature levels can be obtained by sequentially condensing, separating, throttling and evaporating, and according to whether the mixed refrigerant is mixed with the raw natural gas, There are two types of mixed refrigeration processes: closed and open.
(3) Liquefaction process with expander
The expansion refrigeration cycle mostly adopts the Reverse-Brayton cycle. In this cycle, the working fluid is isentropically compressed by the compressor, cooled by the cooler, and then expanded isentropically adiabatically in the turboexpander and does external work to obtain Low temperature airflow to produce cold energy. In the process of natural gas liquefaction, expansion refrigeration mainly adopts the following four forms: natural gas direct expansion refrigeration, nitrogen expansion refrigeration, nitrogen-methane mixed expansion refrigeration, etc.
5. Refrigeration principle and characteristics of liquefaction process with expander
Expander Cycle refers to the process of realizing natural gas liquefaction by using high-pressure refrigerant and Claude cycle refrigeration through adiabatic expansion of a turbo-expander. The key equipment is the turboexpander, which has the advantages of high isentropic efficiency and recoverable expansion work. Therefore, this process is more and more favored by peak-shaving LNG plants with small liquefaction capacity, and is generally used for devices with liquefaction capacity of 7×104～70×104m3/d.
The basic principle of liquefaction process refrigeration with an expander is: the gas expands and cools in the expander while outputting work, which can be used to drive the compressor; when there is a "natural" pressure difference between the raw gas entering the device and the commercial gas leaving the device, the liquefaction The process will not need to be supplemented with energy "from the outside world", but will rely on "natural" pressure differentials to achieve cooling through the expander. According to different refrigerants, it can be divided into nitrogen expansion liquefaction process, nitrogen-methane mixed expansion liquefaction process and natural gas direct expansion liquefaction process.
(1) Natural gas direct expansion liquefaction process
This process refers to the process of directly utilizing the high-pressure natural gas from the gas field and adiabatically expanding it in the expander to the pressure of the transmission pipeline, thereby realizing the process of natural gas liquefaction. It is especially suitable for occasions where the pipeline pressure is high, the actual operating pressure is low, and the pressure needs to be reduced in the middle. Since the natural gas entering the expander does not need to remove CO2, but only needs to remove CO2 from the liquefied part of the raw gas, the pretreatment gas volume is greatly reduced. When the device is in normal operation, the natural gas evaporated from the storage tank is compressed by the return gas compressor and then returned to the system for liquefaction. This process can save the cost of special production, transportation and storage of refrigerant; it has the advantages of simple process, compact equipment, small investment, flexible adjustment and reliable operation. However, this liquefaction process cannot obtain the low temperature, large circulating gas volume and low liquefaction rate as the nitrogen expansion liquefaction process, and the working performance of the expander is greatly affected by the pressure and composition of the raw material gas, and the safety requirements of the system are relatively high. high.
(2) Nitrogen expansion liquefaction process
It is a variant of the direct expansion liquefaction process, the nitrogen refrigeration cycle is separated from the natural gas liquefaction circuit, and the chlorine refrigeration cycle provides cold capacity for the natural gas. Its advantages are that it has greater adaptability to the change of raw gas components, strong liquefaction capacity, simple and convenient operation of the whole system; Lingering agent circulation is about 40% higher.
(3) Nitrogen-methane mixed expansion liquefaction process
It is an improvement of the nitrogen expansion liquefaction process, which can reduce the heat exchange temperature difference at the cold end. Compared with the mixed refrigerant cycle, it has the advantages of simple process, easy control, short start-up time, and saving 10% to 20% of power consumption compared with pure nitrogen expansion refrigeration.
6. The working principle of the turboexpander
A turboexpander is a high-speed rotating thermal machine. According to the law of energy conversion and conservation, when the gas does external work during adiabatic expansion in the turboexpander, its energy will be reduced, and a certain enthalpy drop will be generated at the same time, thereby reducing the temperature of the gas itself and creating conditions for the liquefaction of the gas.
A turboexpander is actually the reverse action of a centrifugal compressor. The centrifugal compressor is driven by an electric motor to increase the pressure of the gas, which consumes power. The turboexpander uses the high-speed airflow generated by the expansion of high-pressure gas to impact the working impeller of the turboexpander, so that the impeller rotates at a high speed. The high-speed rotating impeller can generate a certain amount of power, and then do external work. At the same time, both the temperature and pressure of the expanded gas drop. In other words, the turboexpander uses the speed change of the medium to convert energy, which can not only provide cooling capacity for the liquefaction device, but also the work generated by the expansion can be used to drive equipment such as compressors or generators, reducing the unit of LNG. volumetric energy consumption.
