Foam extraction and removal structure for organic matter solution temporary storage tank

Abstract

The invention discloses a foam extraction and removal structure for an organic matter solution temporary storage tank, which is applied to the related technical field of defoaming. According to the scheme, foam is separated by adopting a physical negative pressure extraction and removal mode in the whole process without adding any chemical defoaming agent, so that the influence of the defoaming agent on the product purity is avoided from the source, and the harsh quality standards of industries such as food and pharmacy can be met; the foam removing process is automatic and continuous, shutdown for manual intervention is not needed, stable operation of a continuous production line is guaranteed, and the utilization rate and the production efficiency of the storage tank are improved; in addition, through the arrangement of the foam gathering assembly, the foam can be gathered towards one side of the foam suction pipe with the holes, so that the extraction operation is carried out in advance, the foam extraction operation does not need to be triggered when the height is accumulated to be higher than that of the foam suction pipe with the holes, and the foam removal operation can be carried out when the height of the foam is relatively low; the amount of foam on the liquid level is always kept in an extremely low state, and the foam removal effect is improved.

Description

Technical Field

[0001] This invention relates to the field of defoaming technology, and in particular to a foam removal structure for a temporary storage tank for organic solutions. Background Technology

[0002] In processes such as post-extraction of fermentation broths containing organic acids like citric acid and amino acids, preparation of traditional Chinese medicine extracts, and pretreatment and temporary storage of high-concentration organic wastewater (e.g., landfill leachate, pharmaceutical wastewater), solutions containing organic matter such as proteins, polysaccharides, surfactants, and microbial residues typically require pH adjustment, temperature control, component homogenization, or simple sedimentation in temporary storage or conditioning tanks before entering core unit operations like evaporation, crystallization, and membrane filtration. During this temporary storage and conditioning process, due to the inherent characteristics of the solution and potential effects such as stirring, pumping circulation, and degradation gas generation, a large, stable, and persistent foam layer easily forms on the liquid surface within the tank.

[0003] The presence of such foam has caused a series of production problems: First, excessive foam will occupy the effective volume of the tank, significantly reducing the actual storage capacity of the tank, forcing the production unit to switch early or reduce the batch processing volume; Second, the foam layer will obscure the true liquid level, causing the control system based on the liquid level gauge (such as the start and stop of the feed pump and discharge pump) to make misjudgments, which may cause equipment cavitation damage due to dry pump operation, or material loss and safety risks due to overflow; Third, when the foam material in the temporary storage tank is pumped to the subsequent precision process (such as evaporator, chromatographic column, precision filter), the gas entrained by the foam will interfere with the heat and mass transfer efficiency, causing fluctuations in the equipment operating point, and even causing problems such as membrane module blockage and chromatographic band distortion, which seriously affect the stability of subsequent processes and product quality.

[0004] To address the foam problem in temporary storage tanks, the current mainstream methods are still to intermittently add chemical defoamers or conduct manual inspections and shut down the machine for cleaning. For example, "silicone" or "polyether" defoamers are commonly used for spraying in fermentation broth storage tanks. For instance, Chinese patent CN118079748B discloses a method and apparatus for preparing an amino polyether organosilicon compound defoamer.

[0005] However, the addition of defoamers undoubtedly introduces exogenous chemical substances into the material system. These substances may adversely affect subsequent catalysis, separation, and purification steps, and leave residues in the final product that are difficult to completely remove, making it difficult to meet the regulatory requirements for high-purity food and pharmaceuticals. Manual cleaning by stopping the machine would disrupt the production process, increase labor costs and intensity, and may also cause material deterioration or contamination risks during cleaning, failing to meet the stable operation requirements of modern continuous and automated production lines. Summary of the Invention

[0006] The core of this invention lies in using a purely physical suction method to remove foam, replacing the addition of defoamers and thus solving the problem of introducing exogenous impurities due to the addition of defoamers in existing technologies. The foam removal process is automatic and continuous, requiring no manual intervention or downtime, ensuring the stable operation of continuous production lines and improving the utilization rate of storage tanks and production efficiency.

[0007] To solve the above problems, the present invention adopts the following technical solution.

[0008] A foam removal structure for an organic solution storage tank includes a bubble monitoring unit installed on the top of the storage tank body and a suction pipe fixedly installed on the outer end of the storage tank body. A controller is installed on the outside of the storage tank body, and the end of the suction pipe extends into the storage tank body and is fixedly connected to a bubble suction tube with holes. A switch valve is installed on the suction pipe.

[0009] The storage tank body is equipped with a bubble-forming component, which includes a bubble-forming ring floating on the liquid surface inside the storage tank body and an electric reel fixedly installed on the inner wall of the storage tank body. The bubble-forming ring includes a strip, a fixed rope fixedly connected to the inner wall of the storage tank body, and two control ropes respectively connected between the two ends of the strip and the electric reel. The fixed rope is located directly below the suction pipe, and the electric reel is located directly above the suction pipe. L-shaped wire holes are drilled at both ends of the fixed rope, and the control ropes move through the corresponding L-shaped wire holes. Multiple evenly distributed buoyancy rods are fixedly connected to the inner and outer walls of the strip. The bubble monitoring unit and the electric reel are both connected to the controller signal.

[0010] Furthermore, the suction pipe is above the liquid surface, the bubble ring floats on the liquid surface, and the liquid surface is not lower than 2 / 3 of the height of the temporary storage tank body.

[0011] Furthermore, multiple buoyancy bars on the inner and outer walls of the strip are interwoven. The buoyancy bars include an outer light shell and fixed strips embedded in the flat surface of the outer light shell. The interior of the outer light shell is saturated with air.

[0012] Furthermore, the outer shell is made of a lightweight sealing material that is resistant to high temperature and high humidity. The outer shell has a semi-cylindrical structure, and the flat surface of the outer shell faces the strip. The fixed strip is fixedly connected to the strip.

[0013] Furthermore, the strip is made of a lightweight elastic material, and in its natural state, the central angle of the strip is 180°-210°.

[0014] Optionally, the strip is made of flexible material, and multiple evenly distributed magnetic sheets are fixedly embedded in the strip. Multiple elastic wires distributed along the circumference of the strip are also embedded inside the strip. The bubble assembly also includes a radial gathering component and an open electromagnetic ring installed on the outside of the temporary storage tank body. When the open electromagnetic ring is energized, it generates a magnetic attraction force on the magnetic sheets.

[0015] Furthermore, the on / off state of the open electromagnetic ring is controlled by the controller, and the central angle corresponding to the open electromagnetic ring is not less than the central angle corresponding to the strip.

[0016] Furthermore, the radial gathering assembly includes a rope gathering ring and multiple radial pull ropes fixedly connected between the outer end of the electric reel and the inner wall of the strip. The multiple radial pull ropes all movably pass through the rope gathering ring, and a positioning rod is fixedly connected between the rope gathering ring and the fixed rope strip.

[0017] Furthermore, the method for foam removal includes the following steps:

[0018] S1. When the bubble monitoring unit at the top of the temporary storage tank detects that the foam height has reached the first threshold, the controller first controls the electric roller to rotate, so that the bubble-gathering ring gathers toward the suction pipe, thereby carrying some foam toward the suction pipe and causing the foam height at the suction pipe to increase locally.

[0019] S2. When the foam height at the suction tube reaches the second threshold due to foam aggregation, the controller controls the vacuum pump connected to the suction tube to start, so that negative pressure is formed in the suction tube, and the foam on the liquid surface is stably sucked away.

[0020] S3. When the foam height at the suction tube is lower than the second threshold, the controller controls the electric roller to rotate in the opposite direction, so that the foam ring gradually resets. If the foam height is still higher than the first threshold at this time, repeat steps S1-S3. If the foam height is lower than the first threshold, stop the foam removal operation.

[0021] Furthermore, the second threshold is greater than the first threshold, and when the foam height is at the first threshold, it is lower than the bottom of the perforated bubble tube, and when the foam height is at the second threshold, it is higher than the bottom of the perforated bubble tube.

[0022] Compared with the prior art, the advantages of this invention are:

[0023] (1) This solution uses physical negative pressure extraction to separate foam throughout the process, without the need to add any chemical defoamer, thus eliminating the impact of defoamer on product purity from the source and meeting the stringent quality standards of the food, pharmaceutical and other industries.

[0024] (2) The foam removal process is automatic and continuous, requiring no manual intervention during shutdown, which ensures the stable operation of the continuous production line and improves the utilization rate and production efficiency of the storage tank.

[0025] (3) The foam removal structure can be directly installed on existing organic solution storage tanks without requiring large-scale modification of the core systems of the original equipment, such as heating, circulation, and condensation.

[0026] (4) By setting up the foam-gathering component, the foam can be gathered toward the side of the bubble tube with holes, thereby increasing the height of the bubbles in that area. This allows for the early removal of foam, meaning that the foam does not need to wait until it accumulates above the bubble tube with holes before the foam removal operation is triggered. Instead, the foam removal operation can be performed when the foam height is relatively low, keeping the amount of foam on the surface of the liquid at a very low level and further improving the foam removal effect. Attached Figure Description

[0027] Figure 1 This is a schematic diagram of the main layout of the present invention;

[0028] Figure 2 This is a perspective view of the suction tube of the present invention;

[0029] Figure 3 This is a perspective view of the temporary storage tank body of the present invention;

[0030] Figure 4 This is a perspective view of the polybubble assembly of the present invention;

[0031] Figure 5 This is a perspective view of the foaming component of the present invention after foam aggregation;

[0032] Figure 6 This is a comparative schematic diagram of the foaming component of the present invention before and after foam aggregation;

[0033] Figure 7 This is a cross-sectional schematic diagram of the buoyancy rod of the present invention;

[0034] Figure 8 A schematic diagram of adding a radial convergence component to the temporary storage tank body of the present invention;

[0035] Figure 9 A schematic diagram of adding a radial convergence component to the polybubble assembly of the present invention;

[0036] Figure 10 A comparative schematic diagram showing the foam aggregation before and after adding a radial gathering component to the foam aggregation component of the present invention.

[0037] Explanation of the labels in the diagram:

[0038] 11 Temporary storage tank body, 2 Suction pipe, 21 Perforated bubble suction pipe, 22 Baffle, 3 Bubble ring, 31 Strip plate, 32 Control rope, 33 Fixed rope strip, 301 L-shaped wire hole, 4 Buoyancy rod, 41 Outer shell, 42 Fixed plate strip, 5 Electric reel, 6 Open electromagnetic ring, 7 Radial gathering assembly, 71 Radial pull rope, 72 Bubble ring, 701 Positioning rod. Detailed Implementation

[0039] The technical solutions will now be clearly and completely described with reference to the accompanying drawings in the embodiments of the present invention.

[0040] First implementation method:

[0041] like Figures 1-2 A foam removal structure for an organic solution storage tank includes a bubble monitoring unit installed on the top of the storage tank body 1 and a suction pipe 2 fixedly installed on the outer end of the storage tank body 1. A controller is installed on the outside of the storage tank body 1. The end of the suction pipe 2 extends into the storage tank body 1 and is fixedly connected to a perforated bubble suction tube 21. A switch valve is installed on the suction pipe 2. Notably, a vacuum pump is connected to the end of the suction pipe 2 through a pipeline. A pressure regulating valve is installed in parallel on the vacuum pump discharge pipeline and the pipeline connecting the vacuum pump and the suction pipe 2. The pressure can be adjusted in real time through the pressure regulating valve to keep the pressure inside the suction pipe 2 stable at a state slightly lower than the pressure inside the storage tank body 1, thereby creating a negative pressure environment to facilitate the suction and removal of foam.

[0042] in, Figure 1 In this diagram, 'a' represents the switching valve, 'b' represents the pressure regulating valve, and 'c' represents the vacuum pump. The bubble monitoring unit is existing technology, and the bubble monitoring unit, vacuum pump, switching valve, and pressure regulating valve are all connected to the controller signal.

[0043] Specifically, the bubble monitoring unit can be set as a sight glass located on the top of the temporary storage tank body 1 and an online video monitoring system installed at the sight glass. The height of the foam on the liquid surface is monitored in real time by the sight glass on the top of the temporary storage tank body 1 combined with the online video monitoring system. When the foam thickness reaches the preset value, the foam removal start signal is triggered.

[0044] The suction pipe 2 is above the liquid surface, the bubble ring 3 floats on the liquid surface, and the liquid surface is not lower than 2 / 3 of the height of the temporary storage tank body 1.

[0045] When the bubble monitoring unit at the top of the temporary storage tank 1 detects that the foam height has reached the preset value, the controller controls the vacuum pump connected to the suction pipe 2 to start, so that a negative pressure is formed in the suction pipe 2, and the foam on the liquid surface is stably sucked out. When the foam height at the suction pipe 2 is lower than the preset value, the foam removal operation stops. If it is higher than the preset value again, the foam removal operation is performed again.

[0046] In summary, this solution uses physical negative pressure extraction to separate foam throughout the entire process, eliminating the need for any chemical defoamers and preventing their impact on product purity from the source. This meets the stringent quality standards of industries such as food and pharmaceuticals. The foam removal process is automatic and continuous, requiring no manual intervention or downtime, ensuring the stable operation of continuous production lines and improving tank utilization and production efficiency. Furthermore, the foam extraction structure can be directly installed on existing storage tanks without requiring large-scale modifications to the original equipment's heating, circulation, and condensation systems, making this foam extraction structure widely applicable.

[0047] Second implementation method:

[0048] This embodiment adds a polybubble component to the first embodiment, while the rest remains the same as the first embodiment.

[0049] like Figures 3-4 The storage tank body 1 is equipped with a bubble-forming component. The bubble-forming component includes a bubble-forming ring 3 floating on the liquid surface inside the storage tank body 1 and an electric reel 5 fixedly installed on the inner wall of the storage tank body 1. The bubble-forming ring 3 includes a strip 31, a fixed rope 33 fixedly connected to the inner wall of the storage tank body 1, and two control ropes 32 respectively connected between the two ends of the strip 31 and the electric reel 5. The fixed rope 33 is located directly below the suction pipe 2, and the electric reel 5 is located directly above the suction pipe 2. L-shaped wire holes 301 are drilled at both ends of the fixed rope 33. The control ropes 32 move through the corresponding L-shaped wire holes 301. Multiple evenly distributed buoyancy rods 4 are fixedly connected to the inner and outer walls of the strip 31. The electric reel 5 is connected to the controller signal.

[0050] Multiple buoyancy rods 4 on the inner and outer walls of the strip 31 are staggered, such as Figure 7 The buoyancy rod 4 includes an outer lightweight shell 41 and a fixed strip 42 embedded in the flat surface of the outer lightweight shell 41. The outer lightweight shell 41 is saturated with air and is made of a lightweight sealing material that is resistant to high temperature and high humidity, which makes the buoyancy rod 4 have a large overall buoyancy. This allows it to carry the entire bubble ring 3 and float stably on the surface of the solution in the storage tank body 1, effectively ensuring that the foam above the liquid surface gathers stably towards the suction pipe 2. The outer lightweight shell 41 has a semi-cylindrical structure and the flat surface of the outer lightweight shell 41 faces the strip 31. The fixed strip 42 is fixedly connected to the strip 31, so the fixed area between the buoyancy rod 4 and the strip 31 is relatively small, making it less likely for the buoyancy rod 4 to restrict the deformation of the bubble ring 3.

[0051] In its natural state, the central angle corresponding to the strip 31 is 180°-210°. The strip 31 is made of lightweight elastic material. When the foam gathers and is sucked out by the perforated suction tube 21, the controller controls the electric roller 5 to rotate in the opposite direction. The control rope 32 loses the restraint of the electric roller 5 and is in a relaxed state. At this time, the control rope 32 is difficult to restrain the two ends of the strip 31. Under the action of the elastic restoring force of the strip 31, the strip 31 gradually expands and resets to facilitate the next bubble gathering operation.

[0052] like Figure 1 The method for removing foam includes the following steps:

[0053] S1. When the bubble monitoring unit at the top of the temporary storage tank 1 detects that the foam height has reached the first threshold, such as... Figures 5-6The controller first controls the electric reel 5 to rotate, thereby winding the control rope 32. Due to the restriction of the fixed rope 33, the control rope 32 generates a pull force on both ends of the strip 31 in the horizontal direction toward the fixed rope 33, thereby causing the fixed rope 33 to gather toward the suction tube 2, thereby carrying some foam to gather at the suction tube 2, causing the foam height at the suction tube 2 to locally increase.

[0054] S2. When the foam height at suction pipe 2 reaches the second threshold due to foam aggregation, the controller controls the vacuum pump connected to the suction pipe to start, so that negative pressure is formed in suction pipe 2, and the foam on the liquid surface is stably sucked away.

[0055] S3. When the foam height at suction tube 2 is lower than the second threshold, the controller controls the electric roller 5 to rotate in the opposite direction, so that the foam ring 3 gradually resets. If the foam height is still higher than the first threshold at this time, repeat steps S1-S3. If the foam height is lower than the first threshold, stop the foam removal operation.

[0056] The second threshold is greater than the first threshold, and when the foam height is at the first threshold, it is lower than the bottom of the perforated bubble tube 21, and when the foam height is at the second threshold, it is higher than the bottom of the perforated bubble tube 21.

[0057] In this embodiment, the second threshold of foam height is the same as the preset foam height value in the first embodiment. This means that in this embodiment, the foam does not need to wait until it accumulates to the preset height value (second threshold) before triggering the foam removal operation. Instead, the foam removal operation can be performed when the foam height is relatively low, so that the amount of foam on the surface of the liquid is always kept at a very low level. Compared with the first embodiment, this further improves the foam removal effect.

[0058] It is worth noting that in this embodiment, the bubble ring 3, buoyancy rod 4, and electric reel 5 are all made of materials resistant to high temperature and high humidity.

[0059] The third implementation method:

[0060] This embodiment is based on the second embodiment, but changes the material of the strip 31 and adds a radially converging component 7, while the rest remains the same as the second embodiment.

[0061] The strip 31 is made of flexible material. Multiple evenly distributed magnetic sheets are fixedly embedded in the strip 31, and multiple elastic wires distributed along the circumference of the strip 31 are embedded inside the strip 31. The elastic wires and the fixed strip 42 can simultaneously enhance the vertical stability of the flexible strip 31 in both longitudinal and vertical directions, making it less prone to large-span collapse, thereby effectively ensuring its foam gathering effect.

[0062] like Figure 8The bubble-gathering assembly also includes a radial gathering assembly 7 and an open electromagnetic ring 6 installed on the outside of the temporary storage tank body 1. When the open electromagnetic ring 6 is energized, it generates a magnetic attraction force on the magnetic sheet. The on and off of the open electromagnetic ring 6 is controlled by the controller, and the central angle corresponding to the open electromagnetic ring 6 is not less than the central angle corresponding to the strip 31. This effectively ensures that after the gathered foam is sucked up, when the open electromagnetic ring 6 is energized, it can fully generate an attraction force on the magnetic sheet to help the strip 31 expand toward the side away from the suction tube 2, so that its distribution range is as large as possible, which is convenient for the next bubble gathering operation.

[0063] like Figure 9 The radial gathering assembly 7 includes a rope-gathering ring 72 and multiple radial pull ropes 71 fixedly connected between the outer end of the electric reel 5 and the inner wall of the strip 31. All radial pull ropes 71 movably pass through the rope-gathering ring 72. A positioning rod 701 is fixedly connected between the rope-gathering ring 72 and the fixed rope strip 33. Figure 10 When the electric reel 5 rotates in the forward direction, the control rope 32 drives the two ends of the strip 31 to converge toward the suction tube 2. At the same time, multiple radial pull ropes 71 can pull the strip 31 toward the suction tube 2 from multiple points in the radial direction. Compared with the second embodiment, a radial converging pulling force is added, which makes the convergence range of the strip 31 larger, improves the foam converging effect, and makes the foam removal effect better.

[0064] The above description is merely a preferred embodiment of the present invention; it encompasses all the protection scope of the present invention. Any equivalent substitutions or modifications made by those skilled in the art within the technical scope disclosed in the present invention, based on the technical solutions and improved concepts of the present invention, should be covered within the protection scope of the present invention.

Claims

1. A foam removal structure for a temporary storage tank of organic solution, comprising a bubble monitoring unit installed on the top of the storage tank body (1) and a suction pipe (2) fixedly installed on the outer end of the storage tank body (1), wherein a controller is installed on the outside of the storage tank body (1), the end of the suction pipe (2) extends into the storage tank body (1) and is fixedly connected to a bubble suction tube (21) with holes, and a switch valve is installed on the suction pipe (2); The storage tank body (1) is equipped with a bubble-forming component. The bubble-forming component includes a bubble-forming ring (3) floating on the liquid surface inside the storage tank body (1) and an electric reel (5) fixedly installed on the inner wall of the storage tank body (1). The bubble-forming ring (3) includes a strip (31), a fixed rope (33) fixedly connected to the inner wall of the storage tank body (1), and two control ropes (32) respectively connected between the two ends of the strip (31) and the electric reel (5). The fixed rope (33) is located directly below the suction pipe (2), and the electric reel (5) is located directly above the suction pipe (2). Both ends of the fixed rope (33) are drilled with L-shaped wire holes (301). The control ropes (32) move through the L-shaped wire holes (301) on the corresponding side. The inner and outer walls of the strip (31) are fixedly connected with multiple evenly distributed buoyancy rods (4). The bubble monitoring unit and the electric reel (5) are both connected to the controller signal.

2. The foam removal structure for an organic solution temporary storage tank according to claim 1, characterized in that: The suction pipe (2) is above the liquid surface, the bubble ring (3) floats on the liquid surface, and the liquid surface is not lower than 2 / 3 of the height of the temporary storage tank body (1).

3. The foam removal structure for an organic solution temporary storage tank according to claim 1, characterized in that: Multiple buoyancy bars (4) on the inner and outer walls of the strip (31) are interleaved. The buoyancy bar (4) includes an outer light shell (41) and a fixed strip (42) fixedly embedded in the flat surface of the outer light shell (41). The outer light shell (41) is saturated with air.

4. The foam removal structure for an organic solution temporary storage tank according to claim 3, characterized in that: The outer shell (41) is made of a lightweight sealing material that is resistant to high temperature and high humidity. The outer shell (41) has a semi-cylindrical structure, and the flat surface of the outer shell (41) faces the strip (31). The fixed strip (42) is fixedly connected to the strip (31).

5. The foam removal structure for an organic solution temporary storage tank according to claim 4, characterized in that: The strip (31) is made of a lightweight elastic material, and in its natural state, the central angle of the strip (31) is 180°-210°.

6. The foam removal structure for an organic solution temporary storage tank according to claim 4, characterized in that: The strip (31) is made of flexible material. Multiple uniformly distributed magnetic sheets are fixedly embedded in the strip (31), and multiple elastic wires distributed along the circumference of the strip (31) are embedded inside the strip (31). The bubble assembly also includes a radial gathering assembly (7) and an open electromagnetic ring (6) installed outside the temporary storage tank body (1). When the open electromagnetic ring (6) is energized, it generates a magnetic attraction force on the magnetic sheets.

7. The foam removal structure for an organic solution temporary storage tank according to claim 6, characterized in that: The opening and closing of the electromagnetic ring (6) is controlled by the controller, and the central angle corresponding to the opening electromagnetic ring (6) is not less than the central angle corresponding to the strip (31).

8. The foam removal structure for an organic solution temporary storage tank according to claim 7, characterized in that: The radial gathering assembly (7) includes a rope loop (72) and a plurality of radial pull ropes (71) fixedly connected between the outer end of the electric reel (5) and the inner wall of the strip (31). The plurality of radial pull ropes (71) all movably pass through the rope loop (72). A positioning rod (701) is fixedly connected between the rope loop (72) and the fixed rope strip (33).

9. The foam removal structure for an organic solution temporary storage tank according to claim 1, characterized in that: The foam removal method includes the following steps: S1. When the bubble height reaches the first threshold detected by the bubble monitoring unit at the top of the temporary storage tank body (1), the controller first controls the electric roller (5) to rotate, so that the bubble-gathering ring (3) gathers towards the suction pipe (2), thereby carrying some foam to gather towards the suction pipe (2), so that the foam height at the suction pipe (2) is locally increased. S2. When the foam height at the suction pipe (2) reaches the second threshold due to the aggregation of foam, the controller controls the vacuum pump connected to the suction pipe (2) to start, so that negative pressure is formed in the suction pipe (2) and the foam on the liquid surface is stably sucked away. S3. When the foam height at the suction tube (2) is lower than the second threshold, the controller controls the electric roller (5) to rotate in the opposite direction, so that the foam ring (3) gradually resets. If the foam height is still higher than the first threshold at this time, repeat steps S1-S3. If the foam height is lower than the first threshold, stop the foam removal operation.

10. The foam removal structure for an organic solution temporary storage tank according to claim 9, characterized in that: The second threshold is greater than the first threshold, and when the foam height is at the first threshold, it is lower than the bottom of the perforated suction tube (21), and when the foam height is at the second threshold, it is higher than the bottom of the perforated suction tube (21).

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