Natural gas liquefaction technology and its applications

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Natural gas liquefaction technology and its applications: Liquefied natural gas (LNG) offers flexible transportation and high storage efficiency. It is useful for expanding urban gas distribution systems and managing peak demand, holding advantages over other methods such as underground gas storage facilities and gas tanks. Additionally, it features low construction costs, a short construction period, rapid implementation, and minimal susceptibility to external factors. As a high-quality fuel for vehicles, compared to conventional gasoline, LNG offers advantages such as a high octane rating, excellent anti-knock properties, complete combustion, reduced exhaust pollution, longer engine life, and lower operating costs. Compared to compressed natural gas (CNG), LNG has the benefits of higher storage efficiency, a longer driving range, lower pressure in storage tanks, lighter weight, fewer storage units required, and no restrictions on site construction imposed by gas supply networks. I. Overview of Natural Gas Liquefaction Technology The natural gas liquefaction system mainly consists of five sub-systems: natural gas pretreatment, liquefaction, storage, transportation, and utilization. The general production process involves purifying natural gas containing over 90% methane through \"three types of removal\" processes (i.e., water removal, hydrocarbon removal, and acidic gas removal), and then using advanced expansion refrigeration techniques or external cooling sources to convert methane into a cryogenic liquid at -162°C. Currently, there are mainly three types of process routes for natural gas liquefaction plants: staged refrigeration process, mixed refrigeration process, and expansion refrigeration process. 1. Staged refrigeration process: The staged refrigeration process is a conventional refrigeration method. In the natural gas liquefaction process, it generally consists of three refrigeration cycle stages using propane, ethylene, and methane as refrigerants, which provide the cooling required for natural gas liquefaction in sequence; the refrigeration temperature gradients are approximately -30°C, -90°C, and -150°C respectively. The purified raw natural gas is cooled, condensed, liquefied, and supercooled step by step in the coolers of 3 refrigeration cycles; after throttling to reduce pressure, a low-temperature and normal-pressure liquid natural gas product is obtained, which is then sent to storage tanks for storage. The staged refrigeration process cooling system is independent of the natural gas liquefaction system; the refrigerant is single-component, with little interaction between the systems, ensuring stable operation, and it is suitable for high-pressure gas sources (by utilizing the pressure energy of the gas source). However, due to the large number of refrigeration units in this process, its long operational sequence, strict requirements regarding the purity of the refrigerant, and its unsuitability for natural gas with high nitrogen content. Therefore, this liquefaction process is less commonly used in natural gas liquefaction plants. 2. Hybrid refrigeration process: The hybrid refrigeration process evolved from the cascade refrigeration process in the late 1960s; it makes use of hydrocarbon mixtures as refrigerants, replacing the multiple pure components used in the cascade refrigeration process. Its refrigerant composition is determined by the composition and pressure of the feed gas; taking advantage of the property that the heavier components in a multi-component mixture condense first while the lighter components condense later, it is condensed, separated, throttled, and evaporated in sequence to produce cooling capacity at different temperature levels. Furthermore, depending on whether the mixed refrigerant is mixed with the raw natural gas, there are two types of mixed refrigeration processes: closed-type and open-type. (1) Closed-loop cycle: The refrigerant circulation system forms an independent system on its own. After being compressed by the refrigeration compressor, the mixed refrigerant is cooled with water (air) and then condensed and separated at different temperatures stage by stage. After throttling, it enters the various temperature zones of the cold box (heat exchanger) to supply cooling capacity to the raw natural gas. After undergoing \"three separations\" treatment, the raw natural gas enters a cold box (heat exchanger), where it is cooled and condensed step by step; through throttling and pressure reduction, liquid natural gas product is obtained. (2) Open-cycle: The raw natural gas, after undergoing \"three-stage separation\" treatment, is mixed with a mixed refrigerant. It flows sequentially through various heat exchangers and gas-liquid separators; as it condenses gradually, the desired refrigerant components are also condensed and separated one by one. These separated refrigerant components are then vaporized in stages according to their boiling points, and together they form a low-temperature fluid stream that undergoes counter-current heat exchange with the raw natural gas in this refrigeration cycle. The open-loop circulation system takes a long time to start, is difficult to operate, and its technology is not yet mature. Compared with the cascade refrigeration process, the hybrid refrigeration process has advantages such as a shorter process flow, fewer units, and lower investment; its disadvantages are higher energy consumption compared to the cascade method, strict requirements for the ratio of the components in the hybrid refrigerant, and greater difficulty in design calculations. 3. Expansion refrigeration process: The characteristic of the expansion refrigeration process is that it utilizes the pressure energy of the raw natural gas to perform work, thereby providing the cooling required for the liquefaction of natural gas. The system liquefaction rate mainly depends on the expansion ratio and expansion efficiency; this process is particularly suitable for gas sources where the transmission pressure is high while the actual usage pressure is low, requiring pressure reduction in between. The advantages are low energy consumption, short process, low investment, and flexible operation; the disadvantage is a low liquefaction rate. II. Current Status of Natural Gas Liquefaction Technology in China 1. The liquefaction process at the Zhongyuan LNG plant: LNG plants in oil fields employ a staged refrigeration process. In view of the high pressure of natural gas sources in the Zhongyuan Oilfield, researchers proposed a process technology scheme based on propane + ethylene + throttling. By collaborating with the experienced French company Sofidra, this process technology scheme was further improved and refined, resulting in lower investment costs, higher yields, and reduced production expenses for the project. The specific process is as follows: High-pressure raw natural gas at 120 bar/27°C (1 bar = 105 Pa) enters the facility, where it is separated into liquid and gas phases in a high-pressure separation tank. It then proceeds to a CO2 removal system that uses MEA as an absorbent, with molecular sieves being used for dehydration. The purified high-pressure gas is pre-cooled to around -30°C using propane, and then throttled to approximately 53 bar/-60°C. After medium-pressure natural gas has had its heavy hydrocarbons removed, it enters a benzene removal system to eliminate trace amounts of benzene; it is subsequently condensed in an ethylene evaporator and throttled to 10 bar/-123°C, resulting in the separation of medium-pressure tail gas and medium-pressure LNG. The medium-pressure LNG is further throttled to around 3 bar/-145°C to produce low-pressure LNG. Both the low-pressure LNG and the low-pressure tail gas are sent to the low-pressure and medium-pressure pipelines after heat recovery, with the low-pressure LNG being stored in tanks as a product. This project mainly includes systems for high-pressure natural gas purification, high-pressure natural gas liquefaction, low-temperature and high-pressure removal of trace benzene from natural gas, and pressurized storage of low-temperature liquid natural gas. The technical features of this device are as follows: (1) It adopts a staged refrigeration process, resulting in low energy consumption. The high pressure of 12 MPa of the raw natural gas is fully utilized, and throttling is carried out at an appropriate temperature, thereby reducing the energy consumption of the plant. This device enables the partial vapor that is generated during the throttling and cooling of high-pressure natural gas to exchange heat with various media at different temperatures in a proper manner, making full use of the low-temperature cooling capacity of this vapor and thereby **reducing the energy consumption of the device**. (2) Pressurized storage of low-temperature liquefied natural gas contributes to improving the plant’s yield. Compared to storage at atmospheric pressure, pressurized storage of LNG results in an increased liquefaction rate to a certain extent. (3) The low-temperature and high-pressure removal technology for trace benzene in natural gas solves the technical challenges associated with benzene removal from natural gas. The isopentane benzene removal technology was first applied in LNG plants, showing good benzene removal efficiency, as well as advantages such as low investment and simple process. 2. The process of the Shanghai Pudong LNG plant (East China Sea Natural Gas Accident Peak Shaving Station) is located in the LNG facility in Shanghai Pudong. This facility is China’s first natural gas liquefaction plant designed for accident peak shaving, with a processing capacity of 10×104 m3/day. It is primarily used to ensure a reliable supply of gas to downstream users in the event that natural gas extraction and transportation at sea (in the East China Sea) has to be halted due to force majeure. The purification process of this facility is the same as that of the Zhongyuan LNG plant, and the liquefaction process employs an integrated cascade liquefaction method. The liquefaction unit of this device includes core components such as a refrigeration compressor unit, a mixed refrigerant separation tower, and an integrated cryogenic tank. The technical features are as follows: (1) The process flow of the device is simple, with a small number of equipment units. (2) An efficient plate-fin cold box is used, which offers high heat exchange efficiency and reduces the space required for the installation. (3) The small number of moving equipment reduces project investment and maintenance costs. 3. Process of the Xinjiang Guanghui LNG Plant: The Xinjiang Guanghui LNG project is located near Shanshan Railway Station in the Tuhua Oil Field. It employs a mixed refrigeration process, with a processing capacity of 150×104 m3/day. The plant consists of several systems such as purification, liquefaction, storage, and transportation, and its target market is primarily the economically developed areas along the middle and lower reaches of the Yangtze River. The specific process is as follows: The raw natural gas at 15 bar, supplied from the gas transmission network, undergoes liquid separation, filtration, and metering before being fed into a pressurization system that raises its pressure to 50 bar. The pressurized raw natural gas then enters a purification system that uses MEA as an absorbent to remove CO2 and H2S. After purification, the gas is dried using a molecular sieve dryer before entering the liquefaction unit. There, the natural gas is cooled to -162°C through condensation, and its pressure is reduced to atmospheric level (1 bar), resulting in low-temperature liquefied natural gas (LNG). The liquefied natural gas is stored in LNG tanks with a capacity of 2×104 m3. The gas source for this LNG plant comes from the Tuhua oil field. Aside from the refrigerant compression units, the process principles of the treatment unit and liquefaction unit are essentially the same as those of the Shanghai Pudong LNG plant. III. Prospects for the application of natural gas liquefaction technology: China’s natural gas resources are primarily found in the central and western regions, while the developed areas along the southeast coast are those with the highest energy consumption. To make rational use of these resources, it is necessary to resolve the conflict between utilization and transportation. “The completion and operation of the West-to-East Gas Pipeline marked the beginning of the widespread use of natural gas in China. However, the enormous investment required for building such long-distance pipelines, the technical challenges posed by geological conditions, and the high costs associated with maintenance and operation all significantly affect and restrict the ability to provide a reliable, economical, and stable supply of gas to end-users. In terms of small and medium-sized cities with rapid economic development, it is difficult to determine the trends in gas consumption in the short and long term, as well as the future scale of gas usage, due to various factors such as social conditions, geographical factors, and economic policies. The economic viability of a gas transmission network is closely linked to the utilization rate of its capacity; whether such a network can generate the expected economic returns is a key concern for all gas suppliers. Therefore, opting for shorter supply routes rather than longer ones has inevitably become an important strategic approach adopted by the vast majority of gas suppliers. As urban gas consumption increases, the need to upgrade and expand the existing pipeline networks inevitably leads to additional costs associated with those existing assets or to repeated investments. This not only causes significant financial losses for various gas suppliers but also raises the additional costs of gas itself, which hinders the development of the gas market and the acquisition of new customers. After some cities are \"gasified,\" fluctuations in domestic gas demand throughout the seasons and failures in the gas transmission networks can lead to irregular or periodic imbalances in gas supply. Building LNG peak-shaving plants (storage and gasification facilities) can play a valuable role in addressing such issues. According to foreign statistics, more than 100 LNG peak-shaving facilities have been built and put into operation in the United States, Japan, and Europe. They save land and capital compared to building above-ground high-pressure gas tanks and underground gas storage facilities, reduce construction time, and are flexible and not restricted by geological conditions. After being liquefied, natural gas is easier to transport in an economical and reliable manner. Special LNG tankers and ships can be used to transport liquefied natural gas over long distances to markets located in remote areas, deserts, offshore oil and gas fields, or new development zones, offering low risks and high adaptability. As the living standards of urban residents improve and there is a shortage of fuel for vehicles, the demand from urban vehicles for safe, economical, environmentally friendly, and reliable fuel continues to rise. Natural gas, with its excellent combustion and emission properties, is increasingly favored by users. Therefore, accelerating the research on the process technologies for natural gas liquefaction plants suitable for China’s conditions, as well as increasing efforts and investment in the research of related application technologies, has become one of the key issues in the field of natural gas utilization and development, offering broad market prospects.