In the BOG recovery process, compression is a key step. BOG is pressurized by different compression methods to meet the needs of subsequent processing and utilization. Common compression methods include the following:
Positive displacement compression
1. Reciprocating compressor
Working principle: The reciprocating compressor converts the rotational motion of the motor into the reciprocating linear motion of the piston through a crank-connecting rod mechanism. In one working cycle, there are four stages: suction, compression, exhaust and expansion. In the suction stage, the piston moves from the top of the cylinder (top dead center) to the bottom (bottom dead center), the volume in the cylinder increases, and the pressure decreases. When the pressure is lower than the pressure in the suction pipe, the suction valve opens and BOG gas is sucked into the cylinder. In the compression stage, the piston moves upward from the bottom dead center, the volume in the cylinder decreases, the BOG gas is compressed, and the pressure and temperature increase. When the pressure increases to exceed the pressure in the exhaust pipe, the exhaust valve opens and enters the exhaust stage. The high-temperature and high-pressure BOG gas is discharged from the cylinder. Finally, in the expansion stage, the piston moves downward from the top dead center, and the high-pressure gas remaining in the cylinder expands, and the pressure decreases, preparing for the next intake.
Features and applicable scenarios: The advantage of reciprocating compressors is that they have a high compression ratio and can compress BOG gas to a very high pressure, which is suitable for recovery processes with high requirements for the final pressure. At the same time, it has strong adaptability to gas and can handle BOG gas of different compositions and properties. In addition, the efficiency of reciprocating compressors performs well at higher compression ratios. However, it also has some disadvantages, such as complex structure and many parts, which lead to high maintenance costs of equipment, and wearing parts need to be replaced regularly. At the same time, reciprocating compressors have large vibrations and noises, requiring special foundation design and vibration and noise reduction measures. Due to these characteristics, reciprocating compressors are suitable for BOG recovery projects with relatively small processing capacity, high compression ratio requirements, and a certain tolerance for equipment maintenance costs, such as the recovery and treatment of a small amount of BOG generated in some small LNG filling stations or industrial production.
2. Rotary compressor
Vane compressor
Working principle: The vane compressor is mainly composed of cylinder, rotor, vane and other components. The rotor is eccentrically installed in the cylinder, and there are several radial slots on the rotor. The vane is installed in the slot and can slide close to the inner wall of the cylinder under the action of centrifugal force and gas pressure. When the rotor rotates under the drive of the motor, the vane rotates with the rotor. During the rotation process, due to the eccentricity of the rotor, the volume between the vane and the inner wall of the cylinder will change periodically. During the suction process, when the vane rotates to a position relative to the suction port, the volume between the vane and the inner wall of the cylinder begins to increase gradually, and the pressure in the cylinder gradually decreases. When the pressure in the cylinder drops to a level lower than the pressure in the suction pipe, the suction valve opens, and the BOG gas enters the cylinder under the action of the pressure difference. As the vane continues to rotate, the volume continues to increase, and more BOG gas is sucked into the cylinder until the volume increases to the maximum value and the suction process ends. During the compression process, as the vane continues to rotate, the volume between the vane and the inner wall of the cylinder begins to gradually decrease. At this time, the BOG gas is compressed, and the pressure and temperature gradually increase. Since the suction valve has been closed when the volume decreases, the gas backflow is prevented, so the BOG gas can only be compressed in the cylinder. As the vane continues to rotate, the volume continues to decrease, and the gas pressure in the cylinder continues to increase. When the pressure rises to exceed the pressure in the exhaust pipe, the exhaust valve opens and enters the exhaust stage, and the high-temperature and high-pressure BOG gas is discharged from the cylinder.
Features and applicable scenarios: The vane compressor has the advantages of relatively simple and compact structure, and the number of its parts is relatively small, which makes the manufacture, installation and maintenance of the equipment relatively convenient and the cost is relatively low. At the same time, the operation of the vane compressor is relatively stable, with less vibration and noise, which has great advantages for some places with high requirements for the working environment, such as LNG filling stations near residential areas or industrial production facilities in cities. In addition, the vane compressor has a high speed and can achieve a high gas flow in a small volume, which is suitable for BOG recovery projects with moderate processing volume. However, there are some limitations of the vane compressor, such as its relatively low compression ratio, which is generally suitable for compression requirements of medium and low pressures. It may not be suitable for some recovery processes that require BOG gas to be compressed to a very high pressure. At the same time, the vane of the vane compressor will produce greater friction and wear between the vane and the inner wall of the cylinder during high-speed operation, which will not only affect the service life of the vane, but also may cause the performance of the compressor to deteriorate, and the vane needs to be inspected and replaced regularly. Due to these characteristics, the vane compressor is suitable for BOG recovery projects with moderate processing volume, low compression ratio requirements, and high requirements for equipment stability and working environment, such as small LNG satellite stations in some cities or medium-volume BOG generated in industrial production and low pressure requirements.
Screw compressor
Working principle: The screw compressor is mainly composed of a pair of intermeshing male and female screws, which are placed in parallel in the compressor casing. The male screw usually has 4-6 convex teeth, and the female screw has 5-7 concave teeth accordingly. When the compressor is working, the motor drives the male screw to rotate. Since the male and female screws mesh with each other, the rotation of the male screw will drive the female screw to rotate in the opposite direction. As the male and female screws rotate, the teeth of the male and female screws stagger with each other, so that the working volume formed between the teeth and the inner wall of the casing changes periodically. During the suction process, when the teeth of the male and female screws turn to the position opposite to the suction port, the volume between the teeth and the inner wall of the casing begins to gradually increase, and the pressure in the casing gradually decreases. When the pressure in the casing drops below the pressure in the suction pipe, the suction valve opens, and BOG gas enters the casing under the action of the pressure difference. As the male and female screws continue to rotate, the volume continues to increase, and more BOG gas is sucked into the casing until the volume increases to the maximum value, and the suction process ends. During the compression process, when the male and female screws continue to rotate, the degree of staggering between the teeth of the male and female screws gradually decreases, and the volume between the teeth and the inner wall of the casing begins to gradually decrease. At this time, the BOG gas is compressed, and the pressure and temperature gradually increase. Since the suction valve is closed when the volume decreases, the backflow of gas is prevented, so the BOG gas can only be compressed in the casing. As the yin and yang screws continue to rotate, the volume continues to decrease, and the gas pressure in the casing continues to increase. When the pressure rises to exceed the pressure in the exhaust pipe, the exhaust valve opens and enters the exhaust stage, and the high-temperature and high-pressure BOG gas is discharged from the casing.
Features and applicable scenarios: Screw compressors have many advantages, which make them widely used in the field of BOG recovery. First of all, the screw compressor has a simple and compact structure, small size, light weight, and small footprint, which has great advantages for some BOG recovery sites with limited space, such as small LNG filling stations or BOG recovery facilities in industrial production workshops. Secondly, the screw compressor runs smoothly, with low vibration and low noise, which is not only conducive to improving the service life of the equipment and reducing the occurrence of equipment failures, but also provides operators with a relatively quiet and comfortable working environment, which meets the requirements of modern industrial production for the working environment. In addition, the screw compressor has a high speed and can process a large amount of BOG gas in a short time. It has a high gas flow rate and processing capacity, and is suitable for BOG recovery projects with large processing volumes, such as large LNG receiving stations or the recovery and treatment of a large amount of BOG generated in industrial production. At the same time, the compression ratio range of the screw compressor is wide. According to different process requirements and the properties of BOG gas, the gear ratio, speed and other parameters of the male and female screws can be adjusted to meet the requirements of different compression ratios. It has strong adaptability and flexibility. However, the screw compressor also has some shortcomings, such as its high processing accuracy requirements, the tooth shape and meshing clearance of the male and female screws need to be strictly controlled, which makes the manufacturing of the equipment difficult and the cost high. At the same time, during the long-term operation of the screw compressor, the tooth surface of the male and female screws will be subject to a certain degree of wear, which will not only affect the compression performance and efficiency of the compressor, but also may cause equipment failures such as leakage. The male and female screws need to be regularly inspected, repaired and replaced, which increases the maintenance cost and workload of the equipment. Due to these characteristics, screw compressors are suitable for BOG recovery projects with large processing capacity, high requirements for equipment stability and working environment, certain range of requirements for compression ratio, and certain tolerance for equipment cost and maintenance, such as large LNG receiving stations, BOG recovery facilities in industrial production parks, etc.
Speed compression
1. Centrifugal compressor
Working principle: The working principle of the centrifugal compressor is based on the centrifugal force generated by the high-speed rotating impeller on the gas. It is mainly composed of components such as impeller, volute, diffuser, air inlet chamber and exhaust chamber. When the compressor is working, BOG gas first enters the compressor through the air inlet chamber, and the function of the air inlet chamber is to enable the gas to flow into the impeller evenly. Then, the gas enters the high-speed rotating impeller, which usually has multiple blades. During the rotation of the impeller, the blades exert force on the gas, causing the gas to rotate with the impeller. Due to the centrifugal force, the gas flows from the center to the edge of the impeller along the direction of the blades in the impeller, and the speed continues to increase, and the pressure of the gas will also increase. After leaving the impeller, the high-speed flowing gas enters the diffuser. The diffuser is a channel with a gradually increasing cross-section, and its function is to convert the kinetic energy of the gas into pressure energy. When the gas flows in the diffuser, the flow rate of the gas gradually decreases due to the gradual increase in the cross-section of the channel. According to the law of conservation of energy, the kinetic energy reduced by the decrease in the gas flow rate will be converted into the pressure energy of the gas, thereby further increasing the pressure of the gas. The gas that has been pressurized by the diffuser enters the volute. The shape of the volute is similar to that of a snail shell. Its function is to collect the gas flowing out of the diffuser and guide the gas to the exhaust chamber. In the volute, the flow rate of the gas will be further reduced and the pressure will also increase. Finally, the gas collected and further pressurized by the volute is discharged from the compressor through the exhaust chamber. At this time, the pressure of the gas has been increased to meet the requirements of subsequent treatment or utilization.
Features and applicable scenarios: Centrifugal compressors have a series of significant characteristics, which make them have important application value in the field of BOG recovery. First, the centrifugal compressor has a large gas flow rate and can process a large amount of BOG gas in a short time. This is because the impeller of the centrifugal compressor rotates at a high speed, which can continuously inhale and compress the gas, and its gas flow rate is usually much larger than that of the positive displacement compressor. Therefore, centrifugal compressors are particularly suitable for BOG recovery projects with large processing volumes, such as large LNG receiving stations, petrochemical enterprises and other places that produce a large amount of BOG. Secondly, the structure of centrifugal compressors is relatively simple and the number of parts is small, which makes the manufacture, installation and maintenance of the equipment relatively convenient and the cost is relatively low. Compared with positive displacement compressors such as reciprocating compressors, centrifugal compressors do not have complex crank-connecting rod mechanisms, pistons and other vulnerable parts, the equipment has higher reliability and stability, less maintenance workload and lower maintenance costs. At the same time, due to the simple structure, the centrifugal compressor is small in size, light in weight and occupies a small area, which is convenient for installation and layout in places with limited space. In addition, the operation of centrifugal compressors is stable, with low vibration and low noise. This is because the center of gravity of the impeller of the centrifugal compressor is relatively stable when it rotates at high speed, and it will not produce large vibrations caused by the reciprocating motion of the piston like a reciprocating compressor. At the same time, the gas flow of the centrifugal compressor is continuous and stable, and it will not produce large pressure fluctuations and noise during the suction, compression and exhaust processes like a positive displacement compressor. Therefore, the low vibration and low noise characteristics of centrifugal compressors are not only conducive to improving the service life and operating efficiency of equipment, but also provide operators with a relatively quiet and comfortable working environment, which meets the requirements of modern industrial production for working environment. However, centrifugal compressors also have some limitations. On the one hand, the compression ratio of centrifugal compressors is relatively low, and they are generally suitable for compression requirements of medium and low pressures. This is because centrifugal compressors mainly rely on the high-speed rotation of the impeller to produce centrifugal force on the gas to achieve gas compression. Its compression process is relatively gentle, unlike reciprocating compressors that can compress the gas multiple times through the reciprocating motion of the piston to achieve a higher compression ratio. Therefore, for some recovery processes that require BOG gas to be compressed to a very high pressure, centrifugal compressors may not be suitable. On the other hand, centrifugal compressors are more sensitive to changes in gas flow. When the gas flow deviates from its design flow, the efficiency of the centrifugal compressor will drop significantly, and even unstable operation phenomena such as surge may occur. Surge is a phenomenon in which the air flow inside the compressor oscillates and reverses due to too small a gas flow or too high a system pressure during operation, causing the compressor to produce violent vibrations and noise, seriously affecting the normal operation and service life of the compressor. Therefore, when using a centrifugal compressor for BOG recovery, it is necessary to ensure that the flow rate of BOG gas is relatively stable, and to reasonably adjust the operating parameters of the compressor according to the flow rate changes of BOG gas, so as to avoid unstable operation phenomena such as surge, and ensure the efficient and stable operation of the compressor. Due to these characteristics, centrifugal compressors are suitable for BOG recovery projects with large processing capacity, high requirements for equipment stability and working environment, not particularly high requirements for compression ratio, and relatively stable BOG gas flow, such as large LNG receiving stations, petrochemical enterprises, etc., which produce a large amount of BOG and have relatively low pressure requirements. In practical applications, the most suitable compression method and compressor type are usually selected according to the specific needs and characteristics of the BOG recovery project, taking into account the advantages and disadvantages of various compressors, so as to achieve efficient recovery and utilization of BOG, while reducing the investment cost, operation cost and maintenance cost of equipment, and improving the economic and social benefits of the project.
