E-book materials ~ Lecture on Natural Gas Chemical Processing Technology (Chapter 8)

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Chapter 8: Physical Processing Technologies for Natural Gas Section 1: Liquefied Natural Gas Technology Physical processing technologies for natural gas include liquefied natural gas (LNG), absorbed natural gas (ANG), as well as the extraction of helium through low-temperature condensation and membrane separation techniques. 1. Overview: Raw natural gas is pre-treated to remove components and impurities such as C5+, H2S, CO2, and water, and then liquefied by deep cooling to –162°C (at atmospheric pressure) to produce LNG. Since the volume of LNG is only 1/625 that of gaseous natural gas, it is suitable for transportation by ship. The LNG industrial system from natural gas fields to end-users is shown in the figure below: Among the total cost of the LNG industrial system, the cost of raw natural gas accounts for 15–20%, the cost of the liquefaction plant (including pre-treatment, liquefaction, loading, etc.) accounts for 30–40%, the cost of LNG transportation accounts for 10–30% (depending on the distance of transportation), and the cost from the receiving station to the end-users accounts for 15–25%. 2. Types of LNG facilities: (1) Peak-shaving type: Natural gas is liquefied and stored to address peak demand for gas, and such facilities are mainly built in areas far from natural gas sources. There are two methods for peak shaving: one is to store natural gas in a gaseous state at high pressure, and the other is to store it in a liquid state at low pressure and low temperature. (2) Base-type: Also known as base-load type units, it is mainly used for the large-scale production of LNG for export or trade. LNG terminal facilities are mostly built in coastal areas to facilitate loading onto ships for shipment to importing countries or regions. (3) Terminal type: Terminal facilities, also known as receiving stations, are used to receive and store large quantities of LNG delivered from offshore LNG carriers. The stored LNG is vaporized and then fed into the pipeline network to supply users. (4) Satellite type: Primarily used for peak shaving; LNG is transported from receiving stations by ships or special tankers and stored there, to be vaporized and used during periods of high gas demand. The facility does not have liquefaction capabilities; it only consists of storage tanks and vaporization equipment. Explanation of the LNG process flow: The liquefaction of natural gas generally involves two stages: natural gas purification (also known as preprocessing) and the liquefaction of natural gas itself, with the refrigeration cycle system being the key component in this process. First, the raw natural gas is purified through \"three removals\" (i.e., dehydration, dehydrocarbonation, and removal of acidic gases) to eliminate the components that are detrimental during the liquefaction process. After that, it enters the high-efficiency heat exchangers of the refrigeration system where its temperature is continuously reduced; butane, propane, ethane, and other compounds are condensed and separated step by step. Finally, by lowering the temperature to around -162°C at atmospheric pressure, LNG product can be obtained. The diagram above is a typical LNG production process flowchart. 4. Natural Gas: The liquefaction process of natural gas can be divided into 3 types based on the refrigeration cycle used, namely the staged refrigeration cycle, the mixed refrigerant cycle, and the expansion mechanism refrigeration cycle. (2) Hybrid refrigeration process: The hybrid refrigeration process typically uses hydrocarbon mixtures (N2, C1, C2, C3, C4, C5) as refrigerants. Taking advantage of the property that the heavier components in such multi-component mixtures condense first while the lighter components condense later, these components are sequentially condensed, separated, throttled, and evaporated 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. The mixed refrigerant process can be divided into a fully mixed refrigerant process and a pre-cooled and mixed refrigerant process. The diagram on the left shows a typical natural gas liquefaction mixed refrigeration process. (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 natural gas liquefaction. The system liquefaction rate mainly depends on the expansion ratio and expansion efficiency. This process is particularly suitable for gas supply scenarios where the transmission pressure is high while the actual usage pressure is low, and pressure reduction is required in between. The advantages are low energy consumption, short process, low investment, and flexible operation ； The disadvantage is the low liquefaction rate. 5. LNG Storage (1) LNG Receiving Stations The processes at LNG receiving stations can be divided into two types, depending on the method used for handling BOG: one is the process of re-condensing the evaporated gas (boiling of gas, BOG), and the other is the process of directly compressing the BOG. (2) LNG storage tanks: Due to the flammability and extremely low temperature of LNG (–162°C), the pressure inside the tanks is required to be between 0.1 and 1 MPa. The evaporation rate of these tanks is generally between 0.04% and 0.2%, while in the case of smaller tanks, this rate can reach 1%. Storage tanks can be divided into above-ground storage tanks and underground storage tanks. 6. Trends in the Global LNG Industry • Safety: While ensuring safety, LNG production facilities are moving toward larger scales in order to reduce energy consumption and improve efficiency, effectiveness, and competitiveness. • Design: In terms of process design, the process layout is optimized, and analytical methods as well as simulation techniques are employed to determine the optimal investment and energy consumption parameters for LNG facilities ； • Optimization of LNG systems is being achieved through the optimization of liquefaction processes and equipment selection, with continuous efforts being made to optimize the entire LNG system ； • Improving the effectiveness and reliability of the equipment is an important way to reduce LNG costs. • Extending the lifespan of the equipment can reduce LNG costs. • LNG production lines are moving toward larger scales • Reducing energy consumption in LNG production: The theoretical minimum energy required to liquefy 1 m3 of natural gas is 0.18–0.21 kWh. 7. Comparison between LNG and natural gas synthetics: LNG is essentially a physical process aimed at converting natural gas into a liquid form for easier transportation. Gas to liquid (GTL) is a chemical process aimed at converting natural gas into paraffins, diesel, and other specialty chemicals. LNG: The technology is well-developed, the market has seen steady growth over the years, and it boasts an excellent safety record. LNG primarily competes with pipeline natural gas in the maritime transportation market. GTL: It is a completely new technology; its products feature excellent environmental performance and high quality, with the transportation fuel sector, especially diesel, as its main market. Both LNG and GTL require substantial capital investment. Investors must consider various factors such as actual investment costs, the scale of natural gas resources, technical risks, and target markets before making a decision. 8. Prospects for the application of natural gas liquefaction technology in China: China’s natural gas resources are mainly 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 resources, it is necessary to resolve the conflict between utilization and transportation. “The completion and operation of the \"West-to-East Gas Transmission\" pipeline marked the beginning of the widespread use of natural gas in China. Building an LNG peak-shaving plant (storage and gasification unit) can serve as an effective means for peak shaving. Liquefied natural gas is easy to transport economically and reliably, with low risks and high adaptability. Therefore, accelerating the research on the process technologies for natural gas liquefaction units suitable for China’s conditions, as well as increasing efforts and investment in research related to these application technologies, has become one of the key issues in the field of natural gas utilization and development, offering broad market prospects. 9. Basic properties of LNG Physical properties of LNG: Ø Main component: Methane Ø Critical temperature: 190.58 K Ø At normal temperatures, it cannot be liquefied by applying pressure; instead, it needs to be pre-treated to remove impurities such as heavier hydrocarbons, sulfides, carbon dioxide, and water, after which it is cooled to -162°C to achieve liquefaction. Ø The main physical properties are shown in Table 1-1: Relative density of the gas, boiling point/°C (at atmospheric pressure), liquid density (g/l) (at boiling point), high calorific value (MJ/m3), and color. The values range as follows: relative density 0.60–0.70; boiling point approximately -162°C; liquid density 430–460 g/l; high calorific value 41.5–45.3 MJ/m3; color – colorless and transparent. Typical composition of LNG: Properties at the bubble point under atmospheric pressure. Composition 1, Composition 2, Composition 3; Mole fractions/%: N2, CH4, C2H6, C3H8, C4H10, C5H12. Molecular weights/(kg/mol): 0.5, 97.5, 1.8, 0.2; Bubble point temperatures/°C: 16.41, -162.6; Densities/(kg/m3): 431.6, 1.79, 93.9, 3.26, 0.69, 0.12, 0.15, 0.09; Molecular weights: 17.07, -165.3; Densities: 448.8, 0.36, 87.20, 8.61, 2.74, 0.42, 0.65, 0.02; Molecular weights: 18.52, -161.3; Density: 468.7. Characteristics of LNG: Ø Low temperature – At atmospheric pressure, the boiling point of LNG is around -162°C. Ø The density ratio of liquid to gas is high; the density of 1 volume of liquefied natural gas is approximately 600 times that of 1 volume of gaseous natural gas. In other words, 1 volume of LNG can be converted into roughly 600 volumes of gas. Ø Flammability: Under normal environmental conditions, in a mixture of natural gas and air, a concentration of natural gas within the range of 5% to 15% (by volume) can lead to ignition; its lowest flammable limit (LEL) is 4%. Safety characteristics of LNG: 1) Combustion characteristics Ø Combustion range: 5%~15%; that is, it will not burn when the volume fraction is below 5% or above 15% ； Ø Auto-ignition temperature: The lowest temperature at which a mixture of flammable gas and air can ignite spontaneously without the presence of an ignition source is known as the auto-ignition temperature. Methane has relatively stable properties; under atmospheric pressure, the average auto-ignition temperature of pure methane is 650°C. Natural gas, whose main component is methane, has a relatively high auto-ignition temperature, while the auto-ignition temperature of LNG varies depending on its composition. Ø Combustion speed: It is the speed at which the flame propagates through the air-gas mixture. Natural gas has a low combustion speed, with its maximum combustion speed being only 0.3 m/s. 2) Low-temperature properties: Thermal insulation – The insulation materials used in LNG systems should have a low thermal conductivity, low density, low moisture absorption and water absorption rates, strong frost resistance, and should not crack at low temperatures. They should also possess good fire resistance, be odorless, resistant to mold growth, harmless to humans, have high mechanical strength, be durable, cost-effective, and easy to install. Ø Evaporation characteristics: LNG is stored as a boiling liquid in insulated tanks; any heat input from the outside environment causes a certain amount of the liquid to evaporate into gas, which is known as Boiling Off Gas (BOG). Under standard conditions, the density of the vapor is 60% that of air. When the LNG pressure drops below its boiling point pressure, a certain amount of liquid evaporates into gas, and at the same time the temperature of the liquid drops to its boiling point at that pressure; this is known as LNG flashing. The treatment of the vapor generated by LNG evaporation due to pressure/temperature changes is a common issue in the storage and transportation of liquefied natural gas. Ø Leakage characteristics: When LNG leaks to the ground, it evaporates rapidly at first; once thermal equilibrium is reached, the evaporation rate stabilizes at a constant level. When LNG leaks into water, intense convective heat transfer occurs; the evaporation rate remains constant over a certain area. As the LNG flows and the leakage area increases, the evaporation rate of the gas continues to rise until it equals the amount of gas that can be produced by the liquid that has leaked out. The leaked LNG enters the atmosphere in a jet form, where it expands and evaporates while mixing violently with the air. Ø Storage characteristics: ü Stratification: LNG is a multi-component mixture, and its density changes due to variations in temperature and composition; these differences in liquid density cause LNG within the storage tank to stratify. ü Rolling: If LNG has stratified, part of the heat absorbed by the upper layer of liquid is used to provide the potential energy required for evaporation at the liquid surface, while the remaining heat raises the temperature of that upper layer of liquid. As evaporation continues, the density of the upper layer of liquid increases while that of the lower layer decreases. When the densities of the two layers become nearly equal, the interface disappears, and the liquids mix rapidly along with significant amounts of liquid evaporating. At this point, the evaporation rate is much higher than normal, resulting in tumbling. ü Rapid Phase Transition (RPT): When two liquids with a large temperature difference come into contact, if the temperature of the hotter liquid is 1.1 times higher than the boiling point of the colder liquid, the temperature of the colder liquid rises very rapidly, and its surface temperature exceeds the temperature at which spontaneous nucleation occurs (i.e., when bubbles form in the liquid). In this process, the colder liquid can generate a large amount of vapor at an explosive rate through a complex chain reaction mechanism in a very short time; this is why RPT occurs when LNG or liquid nitrogen comes into contact with water. 3) Physiological effects: Ø LNG vapor is non-toxic, but inhaling pure LNG vapor can cause rapid loss of consciousness and death within a few minutes ； Humans do not experience any adverse reactions when exposed to an environment with a methane volume fraction of 9%; however, inhaling excessive amounts of natural gas can lead to oxygen deprivation and suffocation. When the volume fraction of natural gas exceeds 50%, it can cause permanent damage to the human body. 10. The LNG industry chain is a comprehensive, capital-intensive and technology-intensive chain that spans the entire process of the natural gas industry. Natural gas extracted from land or offshore oil fields is pre-treated in liquefaction plants before being liquefied. The resulting LNG is shipped to LNG receiving stations for storage under trade contracts, where it is then vaporized and delivered to consumers via pipeline networks. The diagram below shows a schematic of the LNG industry chain. 1) Natural gas development: The natural gas production process involves the extraction of natural gas and certain levels of processing, after which it is transported to liquefaction plants according to its properties and requirements, so as to meet the specifications for feed gas in LNG plants. 2) Liquification: Its main function is to continuously liquefy the feed gas into LNG products. The key steps include: Ø Pretreatment: Removing impurities that were not eliminated during gas field production, such as water, carbon dioxide, sulfur, and thiols. Ø Removal of NGL: Removing NGL from natural gas to meet the specifications and technical requirements of LNG required for liquefaction. Ø Liquidation: The raw gas is cooled and condensed to -162°C using cryogenic refrigerants, turning it into a liquid product. 3) Storage and loading of Ø Liquid natural gas (LNG) liquid products are stored in insulated storage tanks at or near atmospheric pressure; the most common types of such tanks are single-volume tanks, double-volume tanks, and full-volume tanks. 4) Transportation Ø Sea transport of LNG requires specialized transport ships, which store the liquid product in insulated compartments on the LNG ship under normal pressure or near-atmospheric pressure. Some of the LNG evaporates during transportation, and this evaporated gas can be used as fuel for the transport ship. 5) At the receiving station, the ØLNG product is unloaded from the transport ship via the terminal and stored; thereafter, it is vaporized to become pipeline gas, which is delivered to power plants or, through a local distribution network, to end-users as fuel gas. 6) Overview of gas transmission and distribution networks and users 11, as well as natural gas liquefaction technology: Ø Natural gas liquefaction generally involves two processes: natural gas purification and natural gas liquefaction. Ø At atmospheric pressure, liquefying methane requires cooling the temperature to -162°C. To achieve this, it is necessary to remove corrosive substances such as hydrogen sulfide, carbon dioxide, heavy hydrocarbons, water, and mercury from natural gas, as well as impurities that can cause freezing and blockages in equipment and pipelines at low temperatures. After that, the mixture enters a refrigeration cycle where butane, propane, and ethane are successively condensed and separated, resulting in liquefied natural gas as the final product. Natural gas purification: The feed gas for liquefied natural gas projects comes from natural gas produced in oil and gas fields, as well as condensate gas or associated gas from these fields. It contains various impurities such as hydrogen sulfide, carbon dioxide, heavy hydrocarbons, water, and mercury. Pre-treatment is necessary before liquefaction to prevent freezing and blockages in equipment and pipelines due to the presence of carbon dioxide, heavy hydrocarbons, water, and other such substances. ü The table below lists the maximum allowable levels of impurities in the feed gas required for LNG production. Impurity component Allowable content Impurity component Allowable content H2O CO2 H2S COS