A devolatilization unit for a screw extruder and a screw extruder

Abstract

The application discloses a devolatilization unit of a screw extruder and the screw extruder, and belongs to the technical field of polymer material preparation. The devolatilization unit comprises an anti-backflow channel; the anti-backflow channel has an upstream part and a downstream part; the included angle alpha between the line connecting the geometric centers of the upstream part and the downstream part and the horizontal plane satisfies 15 DEG <= alpha <= 75 DEG, and the downstream part is higher than the upstream part in the direction of gravity. The application is favorable for reducing the pollution and thermal degradation of the melt caused by the backflow of the condensate, reducing the gel particles, black spots, stripes and silver wire bubble defects in the product, and improving the appearance quality and the qualified rate of the product.

Description

Technical Field

[0001] This invention relates to the field of devolatilization equipment for screw extruders, specifically to a devolatilization unit for a screw extruder and a screw extruder. Background Technology

[0002] Screw extruders play a crucial role in the continuous production and post-processing of polymer materials, handling key steps such as melting, mixing, and devolatilization. After polymerization or post-processing, polymers using easily condensable monomer systems, such as polymethyl methacrylate (PMMA), polystyrene (PS), and methyl methacrylate-styrene copolymer (MS), still contain unreacted monomers, oligomers, and volatile components like moisture within the melt. If these volatile components remain in the final product, they can cause quality defects such as silver streaks, gas spots, yellowing, or excessive odor. Therefore, screw extruders typically incorporate a devolatilization unit in the barrel, connected to a vacuum system. This negative pressure removes the volatile components from the melt, purifying it and improving its quality.

[0003] Most existing screw extruder devolatilization units employ a vertically upward or vertically penetrating vacuum port structure. The upper part of the vacuum port is connected to the condenser and vacuum pump via vacuum piping. During the devolatilization process, the extracted volatile components condense into a liquid state upon encountering the cooling of the vacuum piping and the inner wall of the vacuum port, forming a condensate.

[0004] The vertically arranged vacuum inlet causes the condensate to flow back downwards under gravity, dripping directly into the polymer melt inside the barrel. After flowing back into the melt, the condensate comes into contact with the high-temperature melt, causing localized overheating or thermal degradation, leading to the formation of gel particles, black spots, and streaks within the product. Simultaneously, the low-boiling-point components brought in by the backflow will re-vaporize during subsequent extrusion stages, resulting in silver streaks and bubbles on the product surface. This condensate backflow phenomenon is particularly pronounced in the devolatilization process of easily condensable volatile components, becoming a major technical obstacle affecting product appearance and yield. Summary of the Invention

[0005] The technical problem to be solved by the present invention is that the volatile components extracted during the operation of the screw extruder condense into condensate on the inner wall of the vacuum port and easily flow back and drip into the polymer melt in the barrel, causing melt contamination, thermal degradation and product appearance defects.

[0006] In a first aspect, this patent provides a devolatilization unit for a screw extruder, including an anti-backflow channel; the anti-backflow channel has an upstream portion and a downstream portion; the angle α between the line connecting the geometric center of the upstream portion and the geometric center of the downstream portion and the horizontal plane satisfies 15°≤α≤75°, and the downstream portion is higher than the upstream portion in the direction of gravity.

[0007] Furthermore, it also includes a flow guide, which is disposed adjacent to the downstream of the downstream portion, and the flow guide is used to guide the condensate to the collection channel; Furthermore, the guide element includes a guide surface, and the angle β between the guide surface and the horizontal plane satisfies 15°≤β≤75°.

[0008] Furthermore, the screw extruder includes a screw and an inner wall. The screw has a rotating shaft that is substantially parallel to the horizontal plane and an extrusion section for extruding the melt to the inner wall. The outer surface of the screw and the inner wall form a melt space for accommodating the melt. The anti-backflow channel communicates with the melt space on the side of the extrusion section that moves downward relative to the anti-backflow channel.

[0009] Furthermore, it also includes a collection channel for discharging the condensate guided by the flow channel outside the devolatilization unit.

[0010] Furthermore, it also includes a devolatilization channel and a spray element, wherein the devolatilization channel is located adjacent to and connected to the anti-backflow channel downstream of the anti-backflow channel; the devolatilization channel includes an anti-backflow inner wall, which is used to guide the condensate away from the anti-backflow channel; Furthermore, the spray element is used to spray spray liquid into the devolatilization channel.

[0011] Furthermore, it also includes a heat exchange channel located outside the devolatilization channel and / or collection channel, the heat exchange channel being used for the passage of the heat transfer medium.

[0012] Furthermore, it also includes a sight glass, which facilitates observation of the devouring channel and / or collection channel; Furthermore, the sight glass is detachably connected to the evaporation channel.

[0013] Furthermore, the area of ​​the viewing mirror is not less than 10 square centimeters, 20 square centimeters, or 50 square centimeters.

[0014] Furthermore, it also includes a vacuum port, which is used to provide negative pressure to the anti-backflow channel; the vacuum port has a vacuum port axis, which is parallel to the horizontal plane.

[0015] Furthermore, it also includes a filter for filtering solids passing through the collection channel.

[0016] Secondly, this patent also relates to a screw extruder, including the aforementioned devolatilization unit.

[0017] Compared with the prior art, the present invention has at least the following beneficial effects: First, this invention defines the inclination of the anti-backflow channel using a horizontal plane as a unified reference. Regardless of whether the barrel is horizontal or vertical, as long as the downstream section is higher than the upstream section in the direction of gravity, and the line connecting the two geometric centers forms a 15-75 degree angle with the horizontal plane, the gas will flow along a defined upward-sloping channel when being extracted. This, combined with the downstream-adjacent guide component and a collection channel independent of the vacuum pipeline, intercepts and discharges the condensate, blocking the path of condensate backflow and contamination of the melt at its source. This reduces contamination and thermal degradation of the melt caused by condensate backflow, reduces defects such as gel particles, black spots, streaks, and silver bubbles inside the product, and improves the product's appearance quality and yield.

[0018] Secondly, compared with the existing solution of setting up a collection chamber or receiving cylinder downstream of the vacuum side to collect and clean the condensate, the present invention blocks the condensate backflow from the source, the device structure is simpler, and eliminates the risk of condensate backflow contaminating the melt due to untimely cleaning of the downstream collection structure.

[0019] Third, the further designed heat exchange channels (i.e., jackets) and spray components can reduce the amount of condensate generated at the source, the filter can prevent material overflow from entering the vacuum system and causing blockage, and the sight glass (quick-release) makes it easy to observe the material overflow and can be operated directly after removal, thus improving the operability and operational stability of the device. Attached Figure Description

[0020] Figure 1 This is a frontal cross-sectional view of the devolatilization unit of this patent; Figure 2 This is a frontal cross-sectional view of the screw extruder of this patent. Figure 3 This is a side view sectional diagram of the screw extruder of this patent.

[0021] In the picture: 100: barrel 110: Screw 111: Screw extrusion section 120: Melt Space 1: Anti-backflow channel 11: Upstream 12: Downstream 13: Airflow guide 2: Deviation Channel 21: Anti-backflow inner wall 3: Collection Channel 4: Heat exchange channel 41: First medium port 42: Second medium port 5: Sight mirror 6: Filter 7: Sprayer components 8: Vacuum port Detailed Implementation

[0022] The present invention will now be described in conjunction with the accompanying drawings and specific embodiments. It should be understood that the following content is for illustrative purposes only and is not intended to limit the scope of protection of the present invention. Any equivalent substitutions or conventional adjustments made to the flow path, equipment form, mixing elements, control methods, or operating parameters under the concept of the present invention should fall within the scope of protection of the present invention. The specific embodiments of the screw extruder devolatilization unit of the present invention will now be described in conjunction with the accompanying drawings.

[0023] In this embodiment, the barrel 100 of the screw extruder is horizontally positioned. A screw 110 is housed within the barrel 100. The screw 110 has a rotating shaft substantially parallel to the horizontal plane and an extrusion section for extruding the melt against the inner wall of the barrel 100. A melt space 120 for accommodating the melt is formed between the outer surface of the screw 110 and the inner wall of the barrel 100. The upstream portion 11 of the anti-backflow channel 1 communicates with the melt space 120 and is located on the side where the screw extrusion section 111 moves downward relative to the anti-backflow channel 1, such that the upstream portion 11 faces the position where the melt is extruded and volatile components are released. In other arrangements, the barrel 100 may also be vertically positioned, obliquely positioned, or arranged in other forms.

[0024] The devolatilization unit is located on the outside of the barrel 100 and includes an anti-backflow channel 1, a flow guide 13, a devolatilization channel 2, a collection channel 3, a heat exchange channel 4, a sight glass 5, a filter 6, a spray element 7, and a vacuum port 8.

[0025] The anti-backflow channel 1 has an upstream section 11 and a downstream section 12 along the gas flow direction. The upstream section 11 is connected to the inner cavity of the barrel 100, and the downstream section 12 is connected to the vacuum pipeline. The angle α between the line connecting the geometric center of the upstream section 11 and the geometric center of the downstream section 12 and the horizontal plane satisfies 15 to 75 degrees, and the downstream section 12 is higher than the upstream section 11 in the direction of gravity, so that the anti-backflow channel 1 forms an upwardly inclined channel that gradually rises from the upstream section 11 to the downstream section 12.

[0026] In this embodiment, the inner diameter of the barrel 100 is 120 mm, the inner diameter of the anti-backflow channel 1 is 40 mm, the included angle α is 35 degrees, the anti-backflow channel 1 is basically straight and obliquely upward, and the length (measured from the geometric center of the upstream part 11 to the downstream part 12) is 100 mm.

[0027] The purpose of devolatilization is to efficiently remove and extract volatile components entrained in high-viscosity melts in gaseous form from the system, while ensuring continuous and controlled forward transport of the melt. Near the anti-backflow channel, screw extruders typically employ special thread elements such as large lead and deep screw channels, intentionally keeping the screw channels in a state of underfill, thus forming a distinct two-phase distribution within the screw channel cross-section: driven by buoyancy, discrete bubbles rise and converge above the melt, forming a continuous gas-phase enrichment zone; the melt itself, under gravity, settles at the bottom of the screw channel and moves axially by the dragging motion of the screw 110. When performing devolatilization in this embodiment, the melt is mainly located inside the barrel 100, with a portion located within the anti-backflow channel. Within the anti-backflow channel or the nearby screw channels, the melt undergoes gas-liquid two-phase gravity separation. The devolatilization channel is angled upwards, aligning with and reinforcing this natural stratification, allowing the vacuum suction system to directly act on the gas-phase enrichment zone, removing volatiles with the shortest path and least resistance. More importantly, this layout helps reduce condensate backflow—the upward-sloping opening allows the gas phase and condensate to be distributed horizontally, and the condensate is more easily separated from the gas phase in the horizontal channel due to gravity. The anti-backflow channel 1 makes the downstream part 12 higher than the upstream part 11 in the direction of gravity and inclined upward along the gas flow direction. The gas phase generated in the channel, which is relatively less prone to condensation, is geometrically guided away from the upper region of the melt from the moment it is generated, which is the structural basis for the establishment of the anti-backflow function.

[0028] It should be noted that the longer the anti-backflow channel 1 is, the less likely the melt is to overflow from it, but it also makes the gas phase more likely to liquefy within it. Therefore, the specific length of the anti-backflow channel 1 can be designed comprehensively based on the melt viscosity, the severity of pressure changes in the system, and the degree of volatile gas phase. Preferably, the anti-backflow channel 1 can be set to 50mm~100mm, 100mm~200mm, or 200mm~500mm. Similarly, the smaller the included angle α, the greater the horizontal component of the anti-backflow channel 1 under the same length, resulting in better condensate separation, but also making it easier for the melt to overflow.

[0029] A guide member 13 is provided adjacent to the downstream of the downstream section 12 in the anti-backflow channel 1. The guide member 13 includes a guide surface, and the angle β between the guide surface and the horizontal plane satisfies 15 to 90 degrees. In this embodiment, β is 60 degrees. The collection channel 3 is connected to the guide member 13 and is independent of the vacuum pipeline. During the devolatilization process, volatile components are extracted by the vacuum port 8 through the vacuum pipeline, and a portion of them condenses into condensate on the inner wall of the anti-backflow channel 1. Since the downstream section 12 is higher than the upstream section 11 in the direction of gravity and the flow channel is inclined upward, the condensate carried to the downstream section 12 by the airflow is intercepted by the adjacent guide member 13 and guided along the guide surface to the collection channel 3. The collection channel 3 independently of the vacuum pipeline discharges the condensate outside the devolatilization unit. Specifically, the interception and diversion by the guide member 13 and the independent discharge by the collection channel 3 together guide the condensate away in a timely and reliable manner, making it less likely for the condensate to accumulate along the upstream section 11 and drip back into the melt, thus eliminating dead zones. In other embodiments, the guiding surface of the guide member 13 may be arc-shaped or stepped in addition to being planar, as long as it can guide the condensate to the collection channel 3.

[0030] The guide member 13 intercepts the condensate generated on the inner wall of the flow channel and directs it to the collection channel 3, allowing the condensate to be diverted and discharged instead of flowing back to the upstream part 11. This interception and diversion effect is determined by the shape and position of the guide member 13 itself, but the starting part of the guide surface is preferably adjacent to the downstream part 12 of the anti-backflow flow channel 1 to timely distribute the condensate. In this embodiment, the inner wall of the devolatilization channel 2 is an anti-backflow inner wall 21. The anti-backflow inner wall 21 is basically planar, and its upper part is relatively far away from the collection channel 3, while its lower part is relatively close to the collection channel 3, so that the condensate on it tends to move away from the anti-backflow channel 1 and the parts in the devolatilization channel 3 where condensate easily accumulates, making it easier for the condensate to enter the collection channel 3. Furthermore, the anti-backflow inner wall 21 guides the condensate to tend to move to the guide member 13. The anti-backflow inner wall 21 can also be any other shape that tends to cause the condensate on it to move away from the anti-backflow channel 1.

[0031] In this embodiment, the collection channel 3 is independent of the vacuum pipeline, leading the condensate and spray liquid out of the devolatilization unit, preventing the condensate or spray liquid from entering the vacuum system and causing contamination or blockage. This is achieved through its independent extraction structure. Preferably, it is configured parallel to the direction of gravity to achieve higher collection efficiency. This embodiment also includes a filter 6, specifically a Y-shaped filter. The filter 6 is located downstream of the collection channel 3 to filter solids discharged with the condensate or spray liquid, preventing blockage of the collection channel 3 and its downstream pipelines, and ensuring a continuous and reliable extraction of condensate. In other embodiments, the filter may also be located inside or upstream of the collection channel 3.

[0032] In this embodiment, the upstream portion 11 of the anti-backflow channel 1 is located on the side of the screw extrusion section 111 that moves downward relative to the anti-backflow channel 1. The upstream portion 11 is directly opposite the position where the melt is extruded and volatile components are released. This arrangement is determined by the relative positional relationship between the upstream portion 11 and the screw extrusion section 111. Even if process fluctuations cause a momentary increase in filling degree, the melt, constrained by gravity and deposited at the bottom, will not actively rise and invade the top opening, thus ensuring the fault tolerance of the process and the reliability of continuous operation. If the vent is placed on the side or bottom, any slight pressure pulsation or overfilling will cause the melt to directly invade or even block the vent channel. The bottom opening will cause the melt to continuously leak under gravity, resulting in complete failure of the devolatilization function. More importantly, in the case of high melt viscosity, the downward-moving screw extrusion section 111 can keep the melt within the anti-backflow channel 1 as much as possible, which can effectively solve the problem of easy material overflow in the inclined anti-backflow channel 1.

[0033] The devolatilization channel 2 is connected to the anti-backflow channel 1 and located downstream of the downstream section 12. A heat exchange channel 4 is provided outside the devolatilization channel 2 and the collection channel 3. The heat exchange channel 4 allows the heat transfer medium to pass through and is used to independently regulate the local temperature of the devolatilization unit, so that the inner wall temperature of the anti-backflow channel 1 is maintained at a level higher than the dew point of the volatile components, thereby reducing the condensation rate and reducing condensate generation. A spray element 7 is provided inside the devolatilization channel 2. The spray element 7 sprays spray liquid into the devolatilization channel 2 to flush away the condensate and guide it to the collection channel 3.

[0034] In this embodiment, a spray element 7 is provided in the devolatilization channel 2. The spray element 7 sprays spray liquid into the devolatilization channel 2 to flush the condensate and guide it to the collection channel 3, thereby improving the discharge efficiency of the condensate.

[0035] The heat exchange channel 4 is used to control the temperature of the devolatilization unit. The heat transfer medium used in the heat exchange channel 4 can be heat transfer oil, cooling water, or a combination thereof, as long as the local temperature can be independently adjusted. In this embodiment, the heat exchange channel 4 maintains the inner wall temperature of the devolatilization channel 2 and the collection channel 3 at a level below the dew point of the volatile components through independent temperature control, allowing the volatile components to condense in a controlled manner. The heat exchange channel 4 specifically includes a jacket located outside the devolatilization channel 2 and the collection channel 3, and a first medium port 41 and a second medium port 42 communicating with the jacket. In other embodiments, the heat exchange channel 4 can also be independently or integrally arranged outside the anti-backflow channel 1.

[0036] A sight glass 5 is installed on the side wall of the devolatilization channel 2. The sight glass 5 facilitates observation of the overflowing material inside the downstream section 12, the guide member 13, and the collection channel 3. The sight glass 5 is detachable, allowing for direct cleaning of overflowing material through its installation opening after removal, without disassembling the vacuum pipeline or stopping the machine to open the cover, thus improving the operability and maintenance convenience of the device. The area of ​​the sight glass 5 is not less than ten square centimeters, preferably not less than twenty square centimeters, and more preferably not less than fifty square centimeters, facilitating the operator to insert tools into the devolatilization channel 2 or other connected channels for inspection and maintenance operations, such as clamping out overflowing melt. Preferably, the sight glass 5 is detachably connected to the devolatilization channel 2, allowing the operator to easily remove and expose the interior of the devolatilization channel 2. In this embodiment, the area of ​​the sight glass 5 is ten square centimeters, and it is a quick-release sight glass. The operator observes the overflowing material inside the downstream section 12, the guide member 13, and the collection channel 3 through the sight glass 5, and adjusts process parameters such as the screw speed, feed rate, and vacuum level accordingly. When overflowing material needs to be handled, the material can be directly cleaned through its installation opening by removing the sight glass 5, without disassembling the vacuum pipeline or stopping the machine to open the cover, thus shortening the cleaning time and reducing the risk of high-temperature burns. The devolatilization channel 2 near the sight glass 5 is inclined upward, which facilitates observation and forms an anti-backflow inner wall 21 to reduce liquid accumulation and avoid accidental polymerization caused by prolonged retention.

[0037] In a preferred embodiment of the present invention, the anti-backflow channel 1, the guide component 13, the collection channel 3, the vacuum port 8, the filter 6, the spray component 7, the heat exchange channel 4, and the sight glass 5 are used in combination. This combination superimposes lifting guidance, interception and diversion, independent extraction, temperature control and generation reduction, and visual maintenance, further improving the reliability of the anti-backflow and the maintainability of the device, making it a more preferred embodiment. The above-mentioned technical features can also be implemented individually or in any combination, and each will exert the aforementioned corresponding beneficial effects.

[0038] Vacuum port 8 is used to provide negative pressure to anti-backflow channel 1. Since the barrel 100 is horizontally positioned, vacuum port 8 has a vacuum port axis parallel to the horizontal plane. In this embodiment, the vacuum level is maintained at 2 kPa (non-limiting example, falling within the range of 500 Pa to 5 kPa). A filter 6 is installed on the collection channel 3 to filter solids passing through the collection channel 3, preventing solids discharged with the condensate from clogging the collection channel 3 and its downstream pipelines. In this embodiment, the axis of vacuum port 8 is parallel to the horizontal plane and the barrel 100, with the suction direction horizontal, avoiding the devolatilization channel 2, collection channel 3, heat exchange channel 4, sight glass 5, and other devices and their connected pipelines, facilitating the arrangement of the vacuum device and pipelines.

[0039] This patent is particularly applicable to devolatilization operations of easily condensable monomer systems such as polymethyl methacrylate (PMMA), polystyrene (PS), and methyl methacrylate-styrene copolymer (MS) initiated by free radical polymerization. Taking polymethyl methacrylate melt as an example, the temperature of the devolatilization barrel 100 can be controlled within the range of 200°C to 260°C, and the vacuum level can be maintained within the range of 500 Pa to 5000 Pa. The above parameters are merely non-limiting examples for ease of implementation.

Claims

1. A devolatilization unit for a screw extruder, characterized in that, It includes an anti-backflow channel; the anti-backflow channel has an upstream section and a downstream section; the angle α between the line connecting the geometric center of the upstream section and the geometric center of the downstream section and the horizontal plane satisfies 15°≤α≤75°, and the downstream section is higher than the upstream section in the direction of gravity.

2. The devolatilization unit according to claim 1, characterized in that, It also includes a flow guide, which is disposed adjacent to the downstream of the downstream portion, and the flow guide is used to guide the condensate to the collection channel; The guide element includes a guide surface, and the angle β between the guide surface and the horizontal plane satisfies 15°≤β≤75°.

3. The devolatilization unit according to claim 1, characterized in that, The screw extruder includes a screw and an inner wall. The screw has a rotating shaft that is substantially parallel to a horizontal plane and an extrusion section for extruding melt to the inner wall. The outer surface of the screw and the inner wall form a melt space for accommodating the melt. The anti-backflow channel communicates with the melt space on the side of the extrusion section that moves downward relative to the anti-backflow channel.

4. The devolatilization unit according to claim 1, characterized in that, It also includes a collection channel for discharging condensate guided by the flow channel outside the devolatilization unit.

5. The devolatilization unit according to claim 1, characterized in that, It also includes a devolatilization channel and a spray element, wherein the devolatilization channel is located adjacent to and connected to the anti-backflow channel downstream of the anti-backflow channel; the devolatilization channel includes an anti-backflow inner wall, which is used to guide the condensate away from the anti-backflow channel; The spraying component is used to spray liquid into the devolatilization channel.

6. The devolatilization unit according to claim 1, characterized in that, It also includes a heat exchange channel located outside the devolatilization channel and / or collection channel, the heat exchange channel being used for the passage of heat transfer medium.

7. The devolatilization unit according to claim 1, characterized in that, It also includes a sight glass that facilitates observation of the devouring channel and / or collection channel; The sight glass is detachably connected to the devouring channel; The area of ​​the viewing mirror is not less than 10 square centimeters, 20 square centimeters, or 50 square centimeters.

8. The devolatilization unit according to claim 1, characterized in that, It also includes a vacuum port, which is used to provide negative pressure to the anti-backflow channel; the vacuum port has a vacuum port axis, which is parallel to the horizontal plane.

9. The devolatilization unit according to claim 1, characterized in that, It also includes a filter for filtering solids passing through the collection channel.

10. A screw extruder, characterized in that, Includes the devolatilization unit as described in any one of claims 1 to 9.

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