Flow-controllable BOG recondensation device and recondensation process

Abstract

The application belongs to the technical field of BOG processing, and particularly discloses a BOG recondensation device with controllable flow and a recondensation process.The BOG recondensation device has excellent precooling function, and the inlet steady flow straight pipe section and the BOG inlet chamber jointly form a shell-and-tube structure, so that BOG can be efficiently precooled before being mixed with LNG, and BOG with a lower temperature is more easily condensed and liquefied when directly contacting with LNG in a gas-liquid mixing condensation chamber, so that the recondensation efficiency is improved;the combination of the multi-hole plate and the hole plug realizes controllable flow adjustment, and improves the anti-interference ability and applicable range of the recondenser;the multi-hole and multi-nozzle design significantly increases the contact area of LNG and BOG in the gas-liquid recondensation mixing chamber, and strengthens the turbulent disturbance between BOG and LNG, so that the BOG recondensation efficiency is greatly improved.The BOG recondensation process can simplify the process flow, fully utilize the pressure energy and cold energy of LNG, and greatly reduce energy consumption, so that the process has good energy-saving and emission-reducing effects.

Description

Technical Field

[0001] This invention belongs to the field of BOG processing technology, and relates to a BOG recondensation device and recondensation process with controllable flow rate. Background Technology

[0002] The recondenser is a key piece of equipment for achieving BOG recondensation. LNG receiving terminals involve multiple processes, such as LNG loading and unloading, LNG storage, in-terminal cold storage circulation, and LNG pressurization and vaporization for external transportation. LNG evaporation is common in these processes, resulting in the generation of large amounts of BOG. ​​The BOG recondensation process in receiving terminals ensures safe storage and transportation, improves resource utilization and energy efficiency, reduces environmental emissions, lowers operating costs, and enhances the company's economic benefits by recovering and reliquefying BOG generated at each stage. Currently, the most widely used BOG recondenser in receiving terminals is the absorption-type packed tower recondenser, which consists of an upper packed section and a lower buffer section. Within the packed section, a large contact area is provided for LNG and BOG to promote BOG recondensation. LNG flowing down from the top of the tower spreads into a large liquid film along the packing surface, while BOG rising from the bottom / flowing down from the top contacts the subcooled LNG. However, the two are difficult to distribute evenly within the packing, resulting in insufficient gas-liquid contact. In the design of recondensers, to address the problem of low BOG recondensation efficiency due to insufficient gas-liquid contact, the packing arrangement is relatively complex, resulting in a large equipment volume, high flow resistance, and high overall investment and maintenance costs. Furthermore, the packing is a consumable and requires replacement and maintenance, further increasing costs. Therefore, packed tower recondensers have a large footprint and high initial investment and operation and maintenance costs.

[0003] Furthermore, the traditional packed tower BOG recondensation process requires the installation of low-pressure pumps, low-pressure compressors, and high-pressure compressors. To address variations in BOG throughput and LNG export volume, this BOG recondensation process is divided into two parts: first, direct pressurization and export using the BOG compressor; and second, condensation of BOG using LNG's cold energy, followed by mixing and pressurization with mainstream LNG before export. This is because traditional packed tower recondensers have poor anti-interference capabilities, limiting the feed ratio and inlet pressure of LNG and BOG. ​​When the LNG export volume is large enough to meet the LNG requirements for BOG recondensation, a portion of the LNG pressurized by the low-pressure LNG pump is sent to the recondenser. The low-pressure compressor then pressurizes the BOG and sends it to the recondenser. The two mix in the recondenser until the BOG is completely condensed, then merge with the remaining LNG flowing out from the low-pressure pump before being pressurized and vaporized by the high-pressure LNG pump for export. When the LNG export volume is small and cannot meet the LNG volume required for BOG recondensation, due to the limitations of the recondenser's operating conditions, some BOG will not be able to enter the recondensation packing tower. To handle this part of BOG, a high-pressure compressor needs to be started to directly pressurize and export it, which will significantly increase the overall energy consumption. Its process is relatively complex, has poor anti-interference ability, and high energy consumption. Summary of the Invention

[0004] The purpose of this invention is to provide a BOG recondensation device with controllable flow rate, which helps to reduce the energy consumption of BOG recondensation and allows for controllable adjustment of the flow rate during the recondensation process, thereby improving the anti-interference capability and adaptability of the recondenser.

[0005] To achieve the above objectives, the present invention adopts the following technical solution:

[0006] A flow-controllable BOG recondensation device includes an LNG inlet and contraction chamber, a BOG inlet chamber, a gas-liquid mixing and condensation chamber, and a diffuser mixing chamber connected sequentially from front to back.

[0007] The LNG inlet and contraction chamber includes an LNG inlet buffer section and an LNG flow controllable adjustment unit; the inlet end of the LNG inlet buffer section is the LNG inlet, and the outlet end of the LNG inlet buffer section is equipped with the LNG flow controllable adjustment unit.

[0008] The LNG flow controllable adjustment unit includes a perforated plate, multiple straight pipe sections for steady flow, and multiple tapered nozzle sections; wherein the perforated plate is installed at the outlet end of the LNG inlet buffer section, and multiple uniformly distributed through holes of the same diameter are provided on its surface;

[0009] Each through hole is connected to a flow-stabilizing straight pipe section, and the flow-stabilizing straight pipe sections are parallel to each other; the outlet end of each flow-stabilizing straight pipe section is connected to a tapered nozzle section.

[0010] The BOG inlet chamber includes a BOG inlet and a BOG buffer precooling section; wherein the BOG inlet is connected to the BOG buffer precooling section, and the BOG buffer precooling section is coaxial with the LNG inlet and the contraction chamber.

[0011] The gas-liquid mixing condenser includes a converging section and an equal-area mixing section; the converging section is a concentric reducer that contracts sequentially from front to back, and its upstream is connected to the BOG buffer precooling section, and its downstream is connected to the equal-area mixing section.

[0012] Each steady-flow straight pipe section starts from the position of the perforated plate, passes through the BOG buffer precooling section axially, and extends to the inlet of the tapered section. The nozzle outlet of each tapered nozzle section is located within the tapered section.

[0013] The perforated plate is also equipped with a plug that matches the through hole, and the LNG flow rate can be adjusted by pulling out or inserting the plug.

[0014] Furthermore, based on the structure of the aforementioned BOG recondensation device with controllable flow rate, this invention also proposes a corresponding BOG recondensation process, which helps to simplify the BOG recondensation process flow, reduce the floor space and one-time investment and operation and maintenance costs, eliminate unnecessary pressurization equipment, make full use of the pressure energy and cold energy of exported LNG, and significantly reduce energy consumption.

[0015] To achieve the above objectives, the present invention adopts the following solution:

[0016] A BOG recondensation process includes the following steps:

[0017] After being extracted from the storage tank, LNG is pressurized by an external booster pump. The pressurized LNG then enters the BOG recondensation unit through the LNG inlet. The LNG first flows through the inlet buffer section, then through the perforated plate into the steady flow straight pipe section, and finally into the converging nozzle section. As the LNG enters the converging nozzle section, the cross-sectional area continuously decreases, the flow velocity continuously increases, and the pressure continuously decreases. At the nozzle outlet, the pressure is lower than the BOG pressure in the station, and under the suction action, the BOG enters the BOG inlet chamber.

[0018] When BOG enters the BOG inlet chamber, a temperature drop occurs in the BOG buffer pre-cooling section. This means the BOG undergoes a pre-cooling process before contacting and mixing with LNG. The LNG is located inside the steady-flow straight pipe section, while the BOG is located outside. Upon entering the BOG inlet chamber, the BOG experiences a significant temperature drop in the area formed by the steady-flow straight pipe section and the BOG inlet chamber (i.e., the BOG buffer pre-cooling section), indicating that the BOG undergoes a substantial pre-cooling process before contacting and mixing with LNG.

[0019] This is because the steady-flow straight tube bundle and the BOG inlet chamber together form a small coiled-tube heat exchanger, in which the lower-temperature LNG flows through the tube side, while the higher-temperature BOG flows through the shell side. This structure significantly improves the heat exchange efficiency between the BOG and the cryogenic LNG, causing the BOG to cool down significantly before mixing with the LNG. When the cooled BOG and cryogenic LNG come into direct contact in the gas-liquid mixing condensation chamber, they will be more easily condensed and liquefied. In other words, the structure formed by the steady-flow straight tube section and the BOG inlet chamber, resembling a small coiled-tube heat exchanger, can significantly reduce the BOG temperature, thereby further improving the recondensation efficiency.

[0020] After the BOG (Bottle-Off Gas) enters the gas-liquid mixing and condensing chamber, it directly contacts and exchanges heat with the LNG flowing out from multiple nozzle outlets. The multi-nozzle design disperses the LNG, greatly increasing the contact area between LNG and BOG, promoting sufficient heat exchange between them. This design can significantly improve re-condensation efficiency. Furthermore, the BOG enters the gas-liquid mixing and condensing chamber via suction. LNG and BOG undergo jet mixing within the chamber, resulting in significant turbulence. This promotes intense mixing and heat exchange between the LNG and BOG, ensuring thorough condensation of the BOG within the chamber.

[0021] Subsequently, the fluid enters the diffuser mixing chamber. As the wall resistance and cross-sectional area increase, the fluid velocity gradually decreases and the pressure gradually increases, forming a reverse pressure gradient along the fluid flow direction. Under the action of the reverse pressure gradient, a large number of vortices appear in the diffuser mixing chamber region, further enhancing the gas-liquid mixing and contact process, so as to promote the further contact and condensation of a small amount of uncondensed BOG in the fluid with LNG. Finally, the LNG flowing out from the BOG recondensation unit outlet is vaporized and transported out via a small vaporizer.

[0022] During BOG recondensation, the flow rate is controllable and adjustable through the plugs on the perforated plate. If the LNG flow rate decreases, the flow of LNG into the BOG recondensation unit is smaller, and the pressure drop at the nozzle outlet is less pronounced. Consequently, the amount of BOG ejected under the current pressure difference decreases. In this case, the plugs are used to block several symmetrically distributed through holes in the perforated plate to reduce the total area of ​​all nozzle outlets, further reducing the pressure at the nozzle outlets and ensuring sufficient ejection of the BOG to be processed. If the BOG processing capacity increases, several symmetrically distributed plugs are removed to increase the total area of ​​the nozzle outlets, thereby ejecting more BOG.

[0023] The present invention has the following advantages:

[0024] As described above, this invention relates to a flow-controllable BOG recondensation device and recondensation process. The flow-controllable BOG recondensation device of this invention achieves efficient pre-cooling of BOG by utilizing a shell-and-tube structure formed by a steady-flow straight pipe section and the BOG inlet chamber before direct contact mixing of LNG and BOG. ​​This combined design helps to further improve recondensation efficiency. Furthermore, the flow rate is controllable by utilizing a combination of perforated plates and plugs, further improving the recondenser's anti-interference capability and applicability. Additionally, this invention employs a multi-hole, multi-nozzle design (i.e., using multiple tapered nozzle sections), significantly increasing the contact area between LNG and BOG in the gas-liquid recondensation mixing chamber, while simultaneously enhancing turbulence between BOG and LNG, greatly improving BOG recondensation efficiency. Moreover, in this invention, the LNG inlet and contraction chamber, BOG inlet chamber, gas-liquid mixing and condensation chamber, and diffuser mixing chamber are processed separately and assembled and connected via flanges, making processing easy, assembly convenient, and facilitating subsequent maintenance. Based on the aforementioned BOG recondensation device with controllable flow rate, this invention also proposes a corresponding BOG recondensation process. Compared to the traditional packed tower recondensation process, this invention's recondensation process fully utilizes the high-pressure BOG directly injected from the exported LNG. Furthermore, apart from the high-pressure LNG export pump, which is essential in the original process, the entire process requires no other power equipment, nor does it require large equipment such as pre-cooling heat exchangers and gas-liquid separators. This new process has a small footprint, a simple flow, extremely low energy consumption, and lower initial investment and maintenance costs, thus achieving better energy-saving and emission-reduction effects. Attached Figure Description

[0025] Figure 1 This is an internal cross-sectional view of the flow-controllable BOG recondenser in Embodiment 1 of the present invention;

[0026] Figure 2 This is a schematic diagram of the LNG flow controllable adjustment unit in Embodiment 1 of the present invention. Figure 2 (a) in the figure is a sectional view of the element; Figure 2 (b) in the diagram is a schematic diagram of the plug structure; Figure 2 (c) in the diagram is a schematic diagram of the perforated plate.

[0027] Figure 3 This is a schematic diagram showing the connection between a single steady-flow straight pipe section and a tapered nozzle section in Embodiment 1 of the present invention;

[0028] Figure 4 This is a flow chart of the recondensation process using a flow-controllable BOG recondensation device in Embodiment 2 of the present invention;

[0029] Among them, I-LNG inlet and contraction chamber, II-BOG inlet chamber, III-gas-liquid mixing and condensation chamber, IV-diffuser mixing chamber;

[0030] 1-LNG inlet, 2-LNG inlet buffer section, 3-perforated plate, 4-stable flow straight pipe section, 5-converging nozzle section, 6-nozzle outlet, 7-BOG inlet, 8-BOG buffer precooling section, 9-converging section, 10-equal area mixing section;

[0031] 11-BOG manifold, 12-flange, 13-through hole, 14-BOG recondenser, 15-LNG vaporizer, 16-hole plug, 17-LNG storage tank, 18-LNG submerged pump, 19-regulating valve, 20-check valve, 21-LNG high-pressure pump. Detailed Implementation

[0032] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments:

[0033] To address the shortcomings of existing BOG recondensers and recondensation processes using packed towers, such as relatively complex processes, high maintenance costs, recondensation efficiency easily affected by operating parameters, poor anti-interference capabilities, and high energy consumption, this invention proposes a flow-controlled BOG recondensation device and process based on direct gas-liquid jet condensation. This method can achieve efficient BOG recondensation while simplifying existing BOG recondensation processes and reducing energy consumption.

[0034] Example 1

[0035] like Figure 1 As shown in the figure, this embodiment describes a flow-controllable BOG recondensation device, which includes an LNG inlet and contraction chamber I, a BOG inlet chamber II, a gas-liquid mixing and condensation chamber III, and a diffuser mixing chamber IV connected sequentially from front to back.

[0036] The LNG inlet and contraction chamber I includes an LNG inlet buffer section 2 and an LNG flow controllable adjustment unit. The LNG inlet buffer section 2 is a cylindrical straight pipe section, with its inlet end being the LNG inlet 1, and its diameter being the same as that of the LNG inlet buffer section 2.

[0037] An LNG flow controllable adjustment unit is installed at the outlet end of LNG inlet buffer section 2, such as... Figure 2 As shown in (a).

[0038] The LNG flow controllable adjustment unit is used to realize the controllable adjustment of the flow rate during the BOG recondensation process. The LNG flow controllable adjustment unit includes a perforated plate 3, multiple steady flow straight pipe sections 4, and multiple tapered nozzle sections 5.

[0039] The perforated plate 3 is installed at the outlet end of the LNG inlet buffer section 2, and multiple evenly distributed through holes 13 are provided on its surface, such as... Figure 2As shown in (c), each through hole 13 has the same diameter and all penetrates the perforated plate 3.

[0040] In this embodiment, the perforated plate 3 is also a circular plate, which can be installed on the LNG inlet buffer section 2.

[0041] like Figure 1 As shown, each through hole 13 is connected to a flow-stabilizing straight pipe section 4, and the flow-stabilizing straight pipe sections 4 are parallel to each other; the outlet end of each flow-stabilizing straight pipe section 4 is connected to a tapered nozzle section 5.

[0042] The term "mutual parallelism" here refers to the fact that each steady-flow straight pipe section 4 is along the axial direction of the BOG recondenser, i.e. Figure 1 The front-to-back direction of the BOG recondenser shown in the figure (e.g.) Figure 1 (As indicated by the middle arrow).

[0043] In this embodiment, the steady-flow straight pipe section 4 is a cylindrical straight pipe section, and the diameter of the straight pipe section is the same as the diameter of the through hole on the perforated plate. The various tapered nozzle sections 5 are also parallel to each other and are all parallel to the axial direction of the BOG recondensation device.

[0044] Each converging nozzle section 5 is a concentric reducer that contracts sequentially from front to back. The end with the larger diameter is connected to the steady flow straight pipe section 4, and the end with the smaller diameter is the nozzle outlet 6, which is the LNG inlet and the outlet of the contraction chamber I.

[0045] The number of straight flow stabilizing pipe sections 4 is equal to the number of tapered nozzle sections 5, and each straight flow stabilizing pipe section 4 is connected to a corresponding tapered nozzle section 5 as a single unit, such as... Figure 3 As shown. Each flow-stabilizing straight pipe section 4 is connected to a corresponding tapered nozzle section 5, and each flow-stabilizing straight pipe section 4 is also connected to a corresponding through hole 13 on the perforated plate 3 as a whole.

[0046] This design connects the orifice plate 3, the straight flow stabilizing pipe section 4, and the tapered nozzle section 5 into a single unit, forming a controllable LNG flow rate regulating unit. The LNG flow rate regulating unit is also equipped with a plug 16, such as... Figure 2 As shown in (a).

[0047] In this embodiment, the contraction angle α of the tapered nozzle section 5 ranges from 11.5° to 14°. Here, the contraction angle α of the tapered nozzle section 5 is the angle between the flow channel outline of the tapered nozzle section and the central axis. Figure 3 As shown.

[0048] The length of the tapered nozzle section 5 is 4-7 times the diameter of the nozzle outlet 6; the diameter of the steady flow straight pipe section 4 is 3-5 times the diameter of the nozzle outlet 6, and the length of the steady flow straight pipe section 4 is 10 times the diameter of the nozzle outlet 6.

[0049] Where n is the number of through holes provided on the perforated plate, which is a natural number in this embodiment, for example, it can be 7.

[0050] The size of the plug 16 is adapted to the through hole 13, and the LNG flow rate can be regulated by pulling out or inserting the plug 16.

[0051] Specifically, the size of nozzle outlet 6 typically determines the LNG flow rate and BOG throughput.

[0052] During the BOG recondensation process, if the LNG outflow rate decreases, the LNG flow into the BOG recondensation unit will be smaller, and the pressure drop at the outlet of the converging nozzle section 5 will decrease, thus reducing the amount of BOG ejected under the current pressure difference.

[0053] At this point, the perforated plate is plugged with plug 16 to block several symmetrically distributed through holes 13, thereby reducing the total area of ​​all nozzle outlets 6 and further reducing the pressure at the nozzle outlets 6, ensuring that the BOG to be processed is fully ejected.

[0054] Conversely, if the BOG throughput increases, several symmetrically distributed orifice plugs 16 on the perforated plate 3 are removed to increase the total area of ​​all nozzle outlets 6, thereby enabling the ejection of more BOG.

[0055] By adjusting the flow area of ​​the steady-flow straight pipe section 4 through the plug 16, and then adjusting the cross-sectional area of ​​the nozzle outlet 6, the flow rate of the BOG recondensation process can be controlled and adjusted.

[0056] The steady-flow straight pipe section 4 and the BOG inlet chamber together form a shell-and-tube structure. This structure has significant heat exchange efficiency in the heat exchange field. Therefore, when the higher-temperature BOG is drawn into the BOG inlet chamber, the BOG will undergo significant heat exchange with the low-temperature LNG in each steady-flow straight pipe section 4, thus pre-cooling before contacting the LNG and significantly reducing the temperature. When the cooled BOG and LNG come into direct contact in the gas-liquid mixing condensation chamber, they will be more easily condensed and liquefied. In other words, the shell-and-tube structure formed by the steady-flow straight pipe section 4 and the BOG inlet chamber helps to further improve the re-condensation efficiency.

[0057] In addition to achieving flow control, the porous design also disperses the LNG. LNG enters the steady-flow straight pipe section 4 and the tapered nozzle section 5 through the porous plate 3, and is ejected as multiple streams through the multi-nozzle outlet 6. The single LNG stream is dispersed into multiple streams, which significantly increases the contact area between BOG and LNG, and also enhances the turbulence intensity between BOG and LNG. All of these further enhance the mixing of LNG and BOG, making BOG easier to condense and liquefy.

[0058] BOG inlet chamber II includes BOG inlet 7 and BOG buffer precooling section 8; wherein, BOG inlet 7 is connected to BOG buffer precooling section 8, and BOG buffer precooling section 8 is coaxial with LNG inlet and contraction chamber I.

[0059] The BOG buffer precooling section 8 is a cylindrical straight pipe section, and its diameter is larger than that of the LNG inlet buffer section 2. The BOG inlet is a cylindrical pipe, and its central axis is perpendicular to the LNG inlet and the contraction chamber axis.

[0060] In a preferred embodiment, there are two BOG inlets 7 that are symmetrically distributed, and the two BOG inlets are of the same size, such as... Figure 1 As shown. Of course, in this embodiment, there can only be one BOG entry point 7, which will not be elaborated here.

[0061] like Figure 1 As shown, the gas-liquid mixing and condensing chamber III includes a tapering section 9 and a mixing section with equal area 10.

[0062] The tapering section 9 is a concentric reducer that contracts sequentially from front to back. Its upstream end is connected to the BOG buffer precooling section 8, and its downstream end is connected to the equal area mixing section 10. The tapering section 9 and the equal area mixing section 10 are connected as one unit.

[0063] In this embodiment, the equal-area mixing section 10 is a cylindrical straight pipe section.

[0064] The ratio of the nozzle outlet diameter 6 to the diameter of the equal-area mixing section 10 is:

[0065] The axial distance between nozzle outlet 6 and the inlet of the equal-area mixing section is equal to the diameter of nozzle outlet 6. times.

[0066] The axial length of the gas-liquid mixing condenser III is 10-13 times the diameter of its equal-area mixing section 10.

[0067] Each steady flow straight pipe section 4 starts from the position of the perforated plate 3, passes through the BOG buffer precooling section 8 axially, and extends to the inlet of the tapering section 9. Each nozzle outlet 6 is located inside the tapering section 9.

[0068] Before LNG and BOG (Boiling Gas) can fully contact, the BOG is pre-cooled. This is because the steady-flow straight pipe section 4 and the BOG inlet chamber together form a small wound-tube heat exchanger. This structure has excellent heat exchange efficiency, giving the BOG re-condensation unit excellent pre-cooling capabilities. The lower-temperature LNG flows through the tube side, while the higher-temperature BOG flows through the shell side. This structure significantly improves the heat exchange efficiency between the BOG and the cryogenic LNG, causing a substantial temperature drop in the BOG before mixing with LNG. This significantly reduces the BOG temperature, allowing for efficient pre-cooling of the BOG before mixing. The lower-temperature BOG is more easily condensed and liquefied when it comes into direct contact with LNG in the gas-liquid mixing condensation chamber. Therefore, this combined design further enhances re-condensation efficiency.

[0069] The diffuser mixing chamber IV is frustum-shaped and gradually expands from front to back. The upstream of the diffuser mixing chamber IV is connected to the equal-area mixing section 10 of the gas-liquid mixing condenser, and the downstream of the diffuser mixing chamber IV is the outlet of the entire recondenser.

[0070] The length of the diffuser mixing chamber satisfies the following formula:

[0071]

[0072] In the formula, θ0 is the angle between the flow channel outline of the diffuser mixing chamber and the central axis, with a value range of 4°-8°; d2 is the diameter of the outlet section of the diffuser mixing chamber; d1 is the diameter of the inlet section of the diffuser mixing chamber; and L is the length of the diffuser mixing chamber.

[0073] The LNG inlet and contraction chamber I, BOG inlet chamber II, gas-liquid mixing and condensation chamber III, and diffuser mixing chamber IV are all coaxially arranged, and are sequentially connected by flanges. This segmented design facilitates later maintenance and further reduces costs.

[0074] In addition, the equipment occupies a small area and is a static device with no additional energy consumption, thus having the effect of energy saving and emission reduction.

[0075] For example, the LNG inlet and contraction chamber I is connected to the BOG inlet chamber II via flange 12 and fastening bolts. Similarly, the BOG inlet chamber II is connected to the gas-liquid mixing and condensation chamber III, and the gas-liquid mixing and condensation chamber III is connected to the diffuser mixing chamber IV via flange 12 and fastening bolts, respectively. Figure 1 As shown.

[0076] In this embodiment, the LNG inlet and contraction chamber I and the BOG inlet chamber II are connected by a nested coaxial connection, which can fully guarantee the sealing performance between the two after they are connected.

[0077] This invention achieves efficient pre-cooling of BOG by utilizing a small heat exchanger consisting of an inlet stabilizing straight pipe section 4 and the BOG inlet chamber before direct contact mixing, which helps to further improve the BOG re-condensation efficiency. Furthermore, the combination of a perforated plate 3 and a plug 16 enables controllable flow adjustment, further improving the recondenser's anti-interference capability and applicability. In addition, the multi-hole, multi-nozzle outlet 6 design greatly disperses LNG, thereby increasing the contact area between LNG and BOG and intensifying the turbulence disturbance between them, thus enhancing LNG-BOG mixing and significantly improving the BOG re-condensation efficiency. Moreover, in this invention, the LNG inlet and contraction chamber I, BOG inlet chamber II, gas-liquid mixing and condensation chamber III, and diffuser mixing chamber IV are processed and assembled separately, making processing easy, assembly convenient, and facilitating subsequent maintenance.

[0078] Example 2

[0079] This embodiment 2 proposes a BOG recondensation process, which employs a flow-controlled BOG recondensation device as described in embodiment 1 above. Figure 4 As shown, LNG is drawn from storage tank 17 by LNG submersible pump 18 and then enters LNG high pressure pump 21 for multi-stage pressurization to reach the high pressure required for external transmission. A portion of the high-pressure LNG enters BOG re-condensation unit 14. BOG enters BOG re-condensation unit 14 from BOG manifold 11. LNG flowing out of BOG re-condensation unit 14 is vaporized and transmitted externally through small vaporizer 15.

[0080] like Figure 4 As shown, the BOG recondensation process in this embodiment specifically includes the following steps:

[0081] After being drawn from storage tank 17 by LNG submersible pump 18, LNG enters LNG high-pressure pump 21 for multi-stage pressurization to reach the high pressure required for external transmission. A portion of the high-pressure LNG goes to the BOG re-condensation device 14 of this invention to achieve BOG re-condensation. After passing through LNG inlet 1, LNG first flows through inlet buffer section 2, then through perforated plate 3 into steady flow straight pipe section 4, and finally into converging nozzle section 5. As LNG enters converging nozzle section 5, the cross-sectional area continuously decreases, the flow velocity continuously increases, and the pressure decreases accordingly. At nozzle outlet 6, the pressure is lower than the BOG pressure in the station, generating a high degree of vacuum. Under the suction action, BOG enters BOG inlet chamber II from BOG manifold 11. When the BOG flows around the outside of the steady-flow straight pipe section 4, in the shell-and-tube structure formed by the steady-flow straight pipe section 4 and the BOG inlet chamber, the lower-temperature LNG and the higher-temperature BOG undergo efficient heat exchange, and the BOG is pre-cooled to a large extent. After the lower-temperature BOG enters the gas-liquid mixing condenser III, it directly contacts the LNG for heat exchange. The design of the multi-hole, multi-nozzle outlet 6 disperses the LNG, greatly increasing the contact area between LNG and BOG, promoting full contact heat exchange between BOG and LNG. Moreover, the BOG and multiple streams of LNG will undergo jet mixing in the gas-liquid mixing condenser III, and there is a large turbulence disturbance between them, which further enhances the mixing and heat exchange between LNG and BOG. ​​Therefore, the BOG will condense quickly and fully in the gas-liquid mixing condenser III region. The fluid then enters the diffuser mixing chamber IV. Due to wall resistance and the increase in cross-sectional area, the fluid velocity gradually decreases while the pressure gradually increases, forming a reverse pressure gradient along the fluid flow direction. Under the influence of this gradient, numerous vortices appear in the region of diffuser mixing chamber IV, further enhancing the gas-liquid mixing and contact process, and causing the small amount of uncondensed BOG in the fluid to completely condense. Finally, the LNG flowing out from the BOG recondenser 14 is vaporized and transported via a small vaporizer 15.

[0082] During daily operation, if the LNG output (flow rate) decreases, the LNG flow rate into the BOG recondenser 14 will be small, and the pressure drop at the nozzle outlet 6 will decrease. This will reduce the amount of BOG ejected under the current pressure difference. In this case, the symmetrically distributed through-holes 13 of the perforated plate 3 will be plugged with plugs 16. Figure 2 As shown, reducing the total area of ​​nozzle outlet 6 further reduces the pressure at nozzle outlet 6, ensuring sufficient entrainment of BOG to be processed. If the LNG export volume increases, the orifice plug 16 can be removed from the perforated plate 3, increasing the total area of ​​all nozzle outlets 6, thereby increasing the entrainment volume of BOG. ​​That is, the present invention can achieve controllable adjustment of the flow rate during BOG recondensation through the combination of the perforated plate 3 and the orifice plug 16.

[0083] If the BOG recondenser malfunctions during operation or requires regular maintenance, the flange 12 connections between the components can be disassembled for detailed inspection and maintenance of the internal structure. If irreversible damage occurs at a location in the recondenser 14, only the damaged component needs to be machined and replaced, further reducing costs. In other words, the segmented design and assembly method of the recondenser proposed in this invention facilitates machining, assembly, and subsequent maintenance.

[0084] The BOG re-condensation process described in this invention involves LNG flowing from the storage tank being pressurized to the export pressure by a multi-stage booster pump. A portion of the LNG is directly vaporized and exported, while the remaining high-pressure LNG enters the aforementioned BOG re-condensation unit 14. The low-pressure BOG is directly drawn in by the suction action of this unit, and the two are vigorously mixed within it, resulting in complete liquefaction of the BOG. ​​Because the diffuser mixing chamber IV of the re-condensation unit proposed in this invention has a backpressure function, the LNG flowing out of the BOG re-condensation unit does not need to be re-pressurized by a booster pump and can directly enter the small LNG vaporizer 15 for vaporization and export. Compared to traditional re-condensation processes, the process is simpler. Apart from the high-pressure LNG export pump, which is essential in traditional processes, no other power equipment is required, especially no compressor, significantly reducing energy consumption.

[0085] The re-condensation process of this invention, employing a flow-controllable BOG re-condensation device, compared to the traditional packed tower re-condensation process, fully utilizes the high pressure of the exported LNG (i.e., fully utilizes the pressure energy and cold energy of LNG) to directly inject the BOG. ​​The entire process requires no additional power equipment, nor large equipment such as pre-cooling heat exchangers and gas-liquid separators, except for the high-pressure LNG export pump essential in the original process, thus significantly reducing energy consumption. The new process described in this embodiment has a small footprint, a simple flow, extremely low energy consumption, and lower initial investment and maintenance costs, resulting in better energy saving and emission reduction effects.

[0086] Of course, the above description is only a preferred embodiment of the present invention. The present invention is not limited to the above-described embodiments. It should be noted that any equivalent substitutions or obvious modifications made by those skilled in the art under the guidance of this specification fall within the scope of this specification and should be protected by the present invention.

Claims

1. A BOG recondensation device with controllable flow rate, characterized in that, It includes, from front to back, the LNG inlet and contraction chamber, the BOG inlet chamber, the gas-liquid mixing and condensation chamber, and the diffuser mixing chamber; The LNG inlet and contraction chamber includes an LNG inlet buffer section and an LNG flow controllable adjustment unit; the inlet end of the LNG inlet buffer section is the LNG inlet, and the outlet end of the LNG inlet buffer section is equipped with the LNG flow controllable adjustment unit. The LNG flow controllable adjustment unit includes a perforated plate, multiple straight pipe sections for steady flow, and multiple tapered nozzle sections; wherein the perforated plate is installed at the outlet end of the LNG inlet buffer section, and multiple uniformly distributed through holes of the same diameter are provided on its surface; Each through hole is connected to a flow-stabilizing straight pipe section, and the flow-stabilizing straight pipe sections are parallel to each other; the outlet end of each flow-stabilizing straight pipe section is connected to a tapered nozzle section. The BOG inlet chamber includes a BOG inlet and a BOG buffer precooling section; wherein the BOG inlet is connected to the BOG buffer precooling section, and the BOG buffer precooling section is coaxial with the LNG inlet and the contraction chamber. The gas-liquid mixing condenser includes a converging section and an equal-area mixing section; the converging section is a concentric reducer that contracts sequentially from front to back, and its upstream is connected to the BOG buffer precooling section, and its downstream is connected to the equal-area mixing section. Each steady-flow straight pipe section starts from the position of the perforated plate, passes through the BOG buffer precooling section axially, and extends to the inlet of the tapered section. The nozzle outlet of each tapered nozzle section is located within the tapered section. The perforated plate is also equipped with a plug that matches the through hole, and the LNG flow rate can be adjusted by pulling out or inserting the plug.

2. The BOG recondensation device with controllable flow rate according to claim 1, characterized in that, The LNG inlet buffer section is a cylindrical straight pipe section, and the BOG buffer precooling section is a cylindrical straight pipe section, with the diameter of the BOG buffer precooling section being larger than the diameter of the LNG inlet buffer section.

3. The BOG recondensation device with controllable flow rate according to claim 1, characterized in that, The BOG inlet is a cylindrical tube with its central axis perpendicular to the LNG inlet and the shrinkage chamber axis; there are two BOG inlets that are symmetrically distributed and have the same size; or there is one BOG inlet.

4. The BOG recondensation device with controllable flow rate according to claim 1, characterized in that, Each converging nozzle section has the same structure and is a concentric reducer that contracts sequentially from front to back. The end with the larger diameter is connected to the steady flow straight pipe section, and the end with the smaller diameter is the nozzle outlet, which is the LNG inlet and the outlet of the contraction chamber.

5. The BOG recondensation device with controllable flow rate according to claim 1, characterized in that, The diffuser mixing chamber is frustum-shaped and gradually expands from front to back. The inlet end of the diffuser mixing chamber is connected to the equal-area mixing section of the gas-liquid mixing and condensing chamber, and the outlet end of the diffuser mixing chamber is the outlet of the BOG re-condensation device.

6. The BOG recondensation device with controllable flow rate according to claim 1, characterized in that, The LNG inlet and contraction chamber, BOG inlet chamber, gas-liquid mixing and condensation chamber, and diffuser mixing chamber are coaxially arranged, and are sequentially connected by flanges.

7. The BOG recondensation device with controllable flow rate according to claim 1, characterized in that, The ratio of the nozzle outlet diameter to the diameter of the equal-area mixing section is: The contraction angle α of the tapered nozzle section ranges from 11.5° to 14°. The length of the tapered nozzle section is 4-7 times the nozzle outlet diameter; The diameter of the straight pipe section for stabilizing the flow is 3-5 times the diameter of the nozzle outlet, and the length is 10 times the diameter of the nozzle outlet. The axial distance between the nozzle outlet and the inlet of the equal-area mixing section is equal to the nozzle outlet diameter. times; The axial length of the gas-liquid mixing and condensing chamber is 10-13 times the diameter of its equal-area mixing section; Where n is the number of through holes set on the perforated plate.

8. The BOG recondensation device with controllable flow rate according to claim 1, characterized in that, The length of the diffuser mixing chamber satisfies the following formula: In the formula, θ0 is the angle between the flow channel outline of the diffuser mixing chamber and the central axis, with a value range of 4°-8°; d2 is the diameter of the outlet section of the diffuser mixing chamber; d1 is the diameter of the inlet section of the diffuser mixing chamber; and L is the length of the diffuser mixing chamber.

9. A BOG recondensation process, based on the flow-controllable BOG recondensation device according to any one of claims 1 to 8, characterized in that, The BOG re-condensation process includes the following steps: After LNG is extracted from the storage tank, it is pressurized by the external booster pump. The pressurized LNG enters the BOG recondenser through the LNG inlet. The LNG first flows through the inlet buffer section, then through the perforated plate into the steady flow straight pipe section, and finally into the converging nozzle section. As LNG enters the converging nozzle section, the cross-sectional area continuously decreases, the flow velocity continuously increases, and the pressure continuously decreases. At the nozzle outlet, the pressure is lower than the BOG pressure inside the station, and under the suction action, the BOG enters the BOG inlet chamber. When BOG enters the BOG inlet chamber, a temperature drop occurs in the BOG buffer pre-cooling section. That is, BOG undergoes a pre-cooling process before it comes into direct contact with LNG for mixing. LNG is located inside the steady flow straight pipe section, while BOG is located outside the steady flow straight pipe section. After BOG enters the gas-liquid mixing and condensing chamber, it directly contacts and exchanges heat with the LNG flowing out from multiple nozzle outlets; LNG and BOG will undergo jet mixing in the gas-liquid mixing and condensing chamber, and BOG will be fully condensed in the gas-liquid mixing and condensing chamber. Subsequently, the fluid enters the diffuser mixing chamber. As the wall resistance and cross-sectional area increase, the fluid velocity gradually decreases and the pressure gradually increases, forming a reverse pressure gradient along the fluid flow direction. Under the action of the reverse pressure gradient, a large number of vortices appear, further enhancing the gas-liquid mixing and contact process, so as to promote the small amount of uncondensed BOG in the fluid to further contact with LNG and condense. Finally, the LNG flowing out of the BOG recondenser is vaporized in a small vaporizer and transported out.

10. The BOG recondensation process according to claim 9, characterized in that, During the BOG recondensation process, the flow rate is controllably adjusted by using plugs on a perforated plate. If the LNG flow rate decreases, the flow rate into the BOG recondensation unit is small, and the pressure drop at the nozzle outlet is reduced, resulting in a smaller amount of BOG ejected under the current pressure difference. In this case, the plugs are used to block several symmetrically distributed through holes in the perforated plate to reduce the total area of ​​all nozzle outlets, further reducing the pressure at the nozzle outlets and ensuring sufficient ejection of the BOG to be processed. If the BOG processing capacity increases, several symmetrically distributed plugs are removed to increase the total area of ​​all nozzle outlets, thereby ejecting more BOG.

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