BOG recovery is mainly based on the difference in physical properties of different substances, and corresponding technical means are used to collect, process and reuse BOG. Common recovery principles include the following:
Principle of using pressure and temperature changes
1. Recondensation process principle
Heat exchange process: The recondensation process is a common method for BOG recovery, which is mainly used in LNG receiving stations and other scenarios. Its principle is based on heat exchange between substances at different temperatures. BOG is generated during the LNG storage process. Due to the introduction of external heat, part of the LNG evaporates to form BOG. The temperature of this part of BOG is relatively high and in a gaseous state. The recondenser introduces low-temperature LNG liquid extracted from the LNG storage tank, and the liquid temperature is usually around -162°C. When BOG and low-temperature LNG liquid are fully in contact in the recondenser, heat will be transferred from the higher temperature BOG to the low-temperature LNG liquid.
BOG liquefaction process: As heat is transferred, the temperature of BOG gradually decreases. When the BOG temperature drops below its dew point temperature, the gaseous molecules in the BOG will gradually gather to form a liquid, thereby realizing the reliquefaction of BOG. After reliquefaction, BOG returns to the LNG liquid state and can be transported back to the LNG storage tank for storage. This not only realizes the recovery of BOG and reduces the impact of BOG emissions on the environment, but also avoids energy waste and improves the energy utilization efficiency during LNG storage and transportation.
2. Compression and condensation principle
BOG compression process: The compression and condensation principle also uses the physical properties of BOG to achieve recovery. First, BOG is compressed by a compressor. BOG is usually in a gaseous state at normal temperature and pressure, and has a large volume. The compressor sucks in and compresses BOG gas through mechanical work, increasing its pressure and reducing its volume. During the compression process, the distance between BOG gas molecules is shortened, and the movement speed of molecules is accelerated, resulting in an increase in the temperature of BOG. This is because the compression process is an adiabatic process, and all the work done by the outside world on the gas is converted into the internal energy of the gas, thereby increasing the temperature of the gas.
Condensation process: After compression, the temperature and pressure of BOG increase significantly. At this time, the high-temperature and high-pressure BOG is introduced into the condenser. The condenser is usually water-cooled or air-cooled, and the heat in the BOG is transferred out by heat exchange with a cooling medium (such as water or air). As the heat is continuously dissipated, the temperature of the BOG gradually decreases. When the temperature of the BOG drops below its dew point temperature, the gaseous molecules in the BOG begin to condense into liquid. Through this compression and condensation method, the originally gaseous BOG is converted into liquid, realizing the recovery of BOG. The recovered liquid BOG can be stored, transported or further processed and utilized according to actual needs. This method is widely used in some small LNG filling stations or industrial production sites because its equipment is relatively simple and easy to operate, which can meet the needs of small-scale BOG recovery.
Principle based on adsorption
1. Adsorption process of BOG by adsorbent
Adsorbent characteristics: Adsorption separation technology is one of the important methods for BOG recovery. Its principle is based on the selective adsorption characteristics of adsorbents on different components in BOG. Commonly used adsorbents such as activated carbon, molecular sieves, activated alumina, etc. have a large specific surface area and rich pore structure. These pores vary in size and range from micropores to mesopores. The larger specific surface area enables the adsorbent to provide more adsorption sites, thereby enhancing the adsorption capacity of various components in BOG. The rich pore structure provides channels for the diffusion and adsorption of gas molecules, so that different components in BOG can selectively enter the pores of the adsorbent according to the differences in their own molecular size and shape, and be adsorbed on the pore surface.
Adsorption process: During the BOG recovery process, the BOG gas is first passed through the pretreatment system to remove impurities such as solid particles and moisture that may be contained in it, so as to avoid these impurities from contaminating or clogging the adsorbent and affecting the adsorption performance and service life of the adsorbent. The pretreated BOG gas enters the adsorption tower and is fully in contact with the adsorbent loaded in the tower. Due to the differences in physical and chemical properties such as molecular size, shape, and polarity, the adsorption capacity of different components in BOG on the surface of the adsorbent is also different. For example, for some small molecular, non-polar or weakly polar gas components, such as methane, due to the relatively good match between its molecular size and the pore size of the adsorbent and the weak interaction force with the adsorbent surface, it is relatively difficult for methane molecules to be adsorbed by the adsorbent during the adsorption process, but it is easier to pass through the pores of the adsorbent and flow out from the outlet of the adsorption tower. On the contrary, for some large molecular and highly polar gas components, such as carbon dioxide, hydrogen sulfide, water, etc., due to their large molecular size and strong interaction force with the adsorbent surface, these gas components are more easily adsorbed by the adsorbent during the adsorption process. They will undergo physical adsorption or chemical adsorption on the adsorbent surface, and thus be trapped in the adsorbent pores. As the adsorption process continues, the adsorption sites on the adsorbent surface are gradually occupied. When the adsorbent reaches the saturated adsorption state, its adsorption capacity for impurity gases in BOG decreases significantly. At this time, the adsorbent needs to be regenerated to restore its adsorption capacity.
2. Adsorbent regeneration principle
Pressure reduction desorption principle: Adsorbent regeneration is the key link to achieve continuous recovery of BOG. Desorption by pressure reduction is one of the common methods for adsorbent regeneration. Its principle is based on the relationship between the adsorption equilibrium of gas on the adsorbent surface and pressure. When the adsorbent adsorbs the impurity gas in BOG under a certain pressure and reaches a saturated adsorption state, there is a dynamic adsorption equilibrium between the gas molecules on the adsorbent surface and the adsorbent. In this equilibrium state, the number of gas molecules adsorbed to the adsorbent surface per unit time is equal to the number of gas molecules desorbed (desorbed) from the adsorbent surface back to the gas phase. When the adsorbent saturated with adsorption is depressurized, the pressure in the gas phase around the adsorbent decreases rapidly. According to the physical and chemical principles of gas adsorption, the reduction in pressure will break the original adsorption equilibrium, so that the number of gas molecules desorbed from the adsorbent surface back to the gas phase is greater than the number of gas molecules adsorbed to the adsorbent surface. In this way, as the pressure reduction process continues, the impurity gas molecules adsorbed on the adsorbent surface will gradually desorb and release into the gas phase, thereby achieving the regeneration of the adsorbent. After desorption and regeneration by pressure reduction, the adsorption sites on the adsorbent surface are released again, and the adsorbent restores its adsorption capacity for impurity gases in BOG, and can be put into the adsorption recovery process of BOG again to realize the recycling of the adsorbent.
Principle of temperature desorption: Temperature desorption is another common adsorbent regeneration method, and its principle is based on the relationship between the adsorption equilibrium of gas on the adsorbent surface and temperature. When the adsorbent adsorbs the impurity gas in BOG at a certain temperature and reaches a saturated adsorption state, there is a dynamic adsorption equilibrium between the gas molecules on the adsorbent surface and the adsorbent. In this equilibrium state, the number of gas molecules adsorbed to the adsorbent surface per unit time is equal to the number of gas molecules desorbed (desorbed) from the adsorbent surface back to the gas phase. When the temperature of the adsorbent saturated with adsorption is increased, the temperature of the adsorbent rises rapidly. According to the physical and chemical principles of gas adsorption, the increase in temperature increases the thermal motion energy of gas molecules, making it easier for gas molecules to overcome the interaction force with the adsorbent surface, thereby desorbing from the adsorbent surface back to the gas phase. In this way, as the temperature rise process continues, the impurity gas molecules adsorbed on the surface of the adsorbent will gradually desorb and release into the gas phase, thereby realizing the regeneration of the adsorbent. After the temperature rise desorption regeneration, the adsorption sites on the surface of the adsorbent are released again, and the adsorbent restores the adsorption capacity of the impurity gas in BOG, and can be put into the adsorption recovery process of BOG again to realize the recycling of the adsorbent. In practical applications, the two methods of pressure reduction desorption and temperature rise desorption can be selected and combined according to factors such as the type of adsorbent, the composition of BOG, and the actual operating conditions, so as to achieve the best adsorbent regeneration effect and BOG recovery efficiency.
Based on the principle of membrane separation
1. The selective permeation process of membranes to different components in BOG
Membrane characteristics: Membrane separation technology has important application value in the field of BOG recovery. Its principle is based on the selective permeation characteristics of special membrane materials to different components in BOG. The membrane materials used for BOG recovery usually have highly ordered microstructures and specific chemical compositions, which give the membrane materials the ability to selectively permeate different gas molecules. There are a large number of nano-scale pores or channels in the microstructure of membrane materials. The size and shape of these pores or channels have a certain matching relationship with the size and shape of different gas molecules. At the same time, the chemical composition of the membrane material determines that its surface has specific chemical properties, such as polarity, charge distribution, etc. These chemical properties affect the interaction between gas molecules and the membrane surface, thereby further affecting the permeation behavior of gas molecules in the membrane.
Selective permeation process: In the BOG recovery process, the BOG gas is first passed through the pretreatment system to remove impurities such as solid particles, moisture, oil droplets, etc. that may be contained therein, so as to avoid these impurities from contaminating, clogging or damaging the membrane material, affecting the separation performance and service life of the membrane. The pretreated BOG gas enters the membrane separation device and contacts the membrane material. In the membrane separation device, the BOG gas diffuses to the membrane surface and interacts with the membrane material under the pressure difference on both sides of the membrane. Since the membrane material has the characteristics of selective permeation to different components in BOG, the permeation behavior of different gas molecules in the membrane is different. For example, for light hydrocarbon gases such as methane, their molecular diameters are relatively small, which is more compatible with the size of the pores or channels in the membrane material, and the interaction between the methane molecules and the membrane surface is weak. Therefore, under the action of the pressure difference on both sides of the membrane, methane molecules are more likely to pass through the pores or channels in the membrane material, permeate from the feed side of the membrane to the permeate side, and form a permeate gas rich in methane. On the contrary, for impurity gases such as carbon dioxide, hydrogen sulfide, and water, their molecular diameters are relatively large, which is less compatible with the size of the pores or channels in the membrane material, and the interaction between these impurity gas molecules and the membrane surface is strong. Therefore, under the action of the pressure difference on both sides of the membrane, these impurity gas molecules are more difficult to pass through the pores or channels in the membrane material, and most of them are trapped on the feed side of the membrane, forming a trapped gas rich in impurity gases. Through the selective permeation process of this membrane to different components in BOG, the separation of methane and impurity gases in BOG is achieved, and the purity of methane in BOG is improved. The recovered methane-rich permeate gas can be stored, transported or further processed according to actual needs, such as used as fuel in power generation, heating and other fields, or as chemical raw materials for the production of synthetic gas, methanol and other chemical products.
2. Influencing factors and optimization measures of membrane separation
Influencing factors
Properties of membrane materials: The chemical composition, microstructure, porosity, pore size distribution and other properties of membrane materials play a decisive role in membrane separation performance. Different membrane materials have different selective permeation capabilities for each component in BOG, which affects the effect of membrane separation and the recovery rate of methane. For example, polymer membrane materials have good flexibility and processing properties, but their chemical stability and high temperature resistance are relatively poor; ceramic membrane materials have high chemical stability, high temperature resistance and mechanical strength, but their preparation process is complex and the cost is high. Therefore, when selecting membrane materials, it is necessary to comprehensively consider factors such as the composition of BOG, operating conditions, separation requirements and cost, and select suitable membrane materials to meet the needs of practical applications.
Operating conditions: Operating conditions such as temperature, pressure, gas flow rate, etc. also have an important influence on the membrane separation process. Changes in temperature will affect the thermal motion energy of gas molecules and the physical and chemical properties of membrane materials, thereby changing the permeation rate and selectivity of gas molecules in the membrane. Generally speaking, within a certain range, an increase in temperature will increase the thermal motion energy of gas molecules, thereby accelerating the permeation rate of gas molecules in the membrane, but it may also cause changes in the physical and chemical properties of the membrane material, such as expansion, contraction or changes in the pore structure of the membrane, thereby affecting the membrane's selective permeability to different gas molecules. Therefore, in actual operation, it is necessary to select a suitable operating temperature based on the properties of the membrane material and the composition of BOG to obtain the best membrane separation effect. Pressure is another important operating parameter in the membrane separation process. The pressure difference on both sides of the membrane is the driving force for the permeation of gas molecules in the membrane. Generally speaking, within a certain range, increasing the pressure difference on both sides of the membrane can increase the permeation rate of gas molecules in the membrane, thereby increasing the membrane flux and the recovery rate of methane. However, excessively high pressure differences may cause damage or deformation of the membrane material, shorten the service life of the membrane, and also increase the investment and operating costs of the equipment. Therefore, in actual operation, it is necessary to reasonably control the pressure difference on both sides of the membrane according to factors such as the properties of the membrane material, the pressure resistance of the equipment, and the economic cost, so as to ensure the stable operation and efficient recovery of the membrane separation process. Gas flow rate is also an important factor affecting the membrane separation process, which will affect the mass transfer rate and residence time of gas molecules on the membrane surface. Generally speaking, within a certain range, increasing the gas flow rate can increase the mass transfer rate of gas molecules on the membrane surface and reduce the concentration polarization phenomenon of gas molecules on the membrane surface, thereby improving the separation performance of the membrane and the recovery rate of methane. However, too high a gas flow rate may cause the penetration time of gas molecules in the membrane to be too short, and the selective permeation of the membrane to different gas molecules cannot be fully realized, thereby reducing the separation effect of the membrane. In addition, too high a gas flow rate will also increase the pressure drop and operating cost of the equipment. Therefore, in actual operation, it is necessary to reasonably control the gas flow rate according to factors such as the properties of the membrane material, the area of the membrane, and the composition of BOG, so as to obtain the best membrane separation effect and economic benefits.
BOG composition: BOG has a complex and diverse composition. The content and proportion of each component in BOG from different sources vary greatly. These differences will directly affect the membrane separation effect and the methane recovery rate. For example, for some BOG containing high concentrations of acidic gases such as carbon dioxide and hydrogen sulfide, these acidic gases will not only corrode and damage the membrane material and shorten the service life of the membrane, but also react chemically with the membrane surface to change the physical and chemical properties of the membrane, thereby affecting the membrane's selective permeability to different gas molecules and reducing the membrane's separation effect and methane recovery rate. In addition, BOG may also contain some impurities such as moisture, oil droplets, and solid particles. These impurities will cause pollution, blockage or wear to the membrane material, affecting the separation performance and service life of the membrane. Therefore, before membrane separation, it is necessary to conduct a detailed analysis of the composition of BOG, and take corresponding pretreatment measures based on the analysis results, such as removing acidic gases, removing moisture and impurities, etc., to improve the quality of BOG, protect membrane materials, and ensure the stable operation and efficient recovery of the membrane separation process.
Optimization measures
Modification and optimization of membrane materials: In order to improve the performance of membrane materials and meet the requirements of membrane separation in different application scenarios, membrane materials can be modified and optimized. Common methods for modifying membrane materials include physical modification, chemical modification and surface modification. The physical modification method mainly improves the separation performance of the membrane by changing the microstructure of the membrane material, such as porosity, pore size distribution, etc. For example, when preparing polymer membranes by phase inversion method, the microstructure of the membrane can be controlled by adjusting the composition, temperature, humidity of the casting liquid and the conditions of the gel bath, so as to obtain membrane materials with different porosities and pore size distributions to meet the requirements of different separation systems for membranes. The chemical modification method mainly introduces specific functional groups into the molecular chain of the membrane material to change the chemical properties of the membrane material, thereby improving the separation performance and anti-fouling ability of the membrane. For example, for polysulfone (PSF) membrane materials, sulfonic acid groups (-SO3H) can be introduced into their molecular chains through sulfonation reaction to prepare sulfonated polysulfone (SPSF) membrane materials. The introduction of sulfonic acid groups not only increases the hydrophilicity of the membrane material, making it easier to interact with water molecules, thereby increasing the membrane's permeation flux to water, but also enhances the membrane material's adsorption and repulsion of certain charged particles or polar molecules, thereby improving the membrane's separation performance and anti-pollution ability. The surface modification method mainly changes the physical and chemical properties of the membrane surface, such as surface roughness, surface energy, surface charge distribution, etc., by treating the surface of the membrane material, thereby improving the membrane's separation performance and anti-pollution ability. For example, for ceramic membrane materials, a layer of thin films with specific functions, such as hydrophilic films, anti-pollution films, and selective permeation films, can be prepared on its surface by methods such as chemical vapor deposition (CVD), physical vapor deposition (PVD), and sol-gel. The introduction of these thin films can not only change the physical and chemical properties of the membrane surface, improve the membrane's separation performance and anti-pollution ability, but also protect the bulk structure of the membrane material to a certain extent and extend the membrane's service life. By modifying and optimizing the membrane material, a membrane material with better separation performance, anti-pollution ability and stability can be obtained, so as to meet the requirements of membrane separation in different application scenarios and improve the efficiency and quality of BOG recovery.
Optimization of operating conditions: Optimization of operating conditions is a key link in improving the efficiency and stability of membrane separation. In actual operation, it is necessary to optimize operating conditions such as temperature, pressure, and gas flow rate according to factors such as the properties of the membrane material, the composition of BOG, and separation requirements to obtain the best membrane separation effect. For temperature optimization, it is necessary to comprehensively consider factors such as the thermal stability of the membrane material, the permeation behavior of gas molecules in the membrane, and the selective permeability of the membrane to different gas molecules. Generally speaking, within the thermal stability range of the membrane material, appropriately increasing the operating temperature can accelerate the permeation rate of gas molecules in the membrane and increase the flux of the membrane, but at the same time, it is also necessary to avoid excessively high temperatures that cause changes in the physical and chemical properties of the membrane material and affect the selective permeability of the membrane. Therefore, in actual operation, it is necessary to determine the optimal operating temperature range suitable for the membrane material and the composition of BOG through experiments or simulation calculations. For the optimization of pressure, factors such as the pressure resistance of membrane materials, the safe operation of equipment, and the effect of membrane separation need to be considered. Under the premise of ensuring the safe operation of membrane materials and equipment, appropriately increasing the pressure difference on both sides of the membrane can increase the permeation rate of gas molecules in the membrane, increase the flux of the membrane and the recovery rate of methane. However, too high a pressure difference may cause damage or deformation of the membrane material, shorten the service life of the membrane, and also increase the investment and operating costs of the equipment. Therefore, in actual operation, it is necessary to reasonably control the pressure difference on both sides of the membrane according to factors such as the properties of the membrane material, the pressure resistance of the equipment, and the economic cost, and determine the optimal operating pressure range. For the optimization of gas flow rate, factors such as the mass transfer rate of gas molecules on the membrane surface, the residence time, and the selective permeability of the membrane to different gas molecules need to be considered. Within a certain range, increasing the gas flow rate can increase the mass transfer rate of gas molecules on the membrane surface and reduce the concentration polarization phenomenon of gas molecules on the membrane surface, thereby improving the separation performance of the membrane and the recovery rate of methane. However, too high a gas flow rate may cause the permeation time of gas molecules in the membrane to be too short, and the selective permeation of the membrane to different gas molecules cannot be fully realized, thereby reducing the separation effect of the membrane. In addition, too high a gas flow rate will also increase the pressure drop and operating cost of the equipment. Therefore, in actual operation, it is necessary to reasonably control the gas flow rate and determine the optimal operating gas flow range based on factors such as the properties of the membrane material, the area of the membrane, and the composition of BOG.
