A co-rotating and counter-rotating flash devolatilization twin-screw extruder

Abstract

This invention discloses a co-directional and counter-directional flash devolatilization twin-screw extruder. The extruder includes a system for creating high pressure and instantaneous release. The system for creating high pressure and instantaneous release includes a flash threaded block element, a small-lead conveying threaded block, a medium-lead conveying threaded block, and a large-lead conveying threaded block connected to the flash threaded block element, as well as a matching large-lead conveying element for depressurization devolatilization, a screw barrel, and a vacuum system. The beneficial effect is that this invention fully utilizes the flash principle, first breaking the original phase equilibrium of the polymer solution to separate it, and at the same time transferring the high energy released instantaneously under high pressure to the volatiles, enabling them to break through the constraints of the high-viscosity polymer melt and burst out. It not only retains the surface renewal and vacuum devolatilization characteristics of conventional twin-screw devolatilization extruders, but also avoids the drawbacks of easy clogging of the devolatilization port and shearing degradation of polymer molecular chains.

Description

Technical Field

[0001] This invention relates to the field of chemical process technology, and in particular to a twin-screw extruder for co-rotating and counter-rotating flash devouring. Background Technology

[0002] Deviation is a widely used process in chemical engineering, and in recent years it has been increasingly applied to the polymer field. For example, in solution polymerization systems, it is essential to completely remove residual solvents and unpolymerized monomers after polymerization; otherwise, the polymer's performance and resource recovery will be severely affected. Most notably, my country produces over 1.5 million tons annually of anionic polymers such as thermoplastic elastomers (SBS, SIS) and K resins, requiring the removal of solvents and residual monomers from these products. Bulk polymerization processes in my country also include polystyrene and styrene / maleic anhydride copolymers (SMA). Annual production reaches approximately 5 million tons, and residual monomers must be removed from the products. This is especially true for propylene and octene copolymer resin (POE), which has recently seen large-scale production lines in my country, reaching 13 production lines with a capacity of 2.15 million tons per year by 2023. Solvents, residual monomers, and waste gases must also be removed from the products. Furthermore, my country's annual polyester resin production reaches nearly 80 million tons, requiring the removal of byproduct water from the polymer to increase its molecular weight. Clearly, all these products rely on devolatilization processes, and these processes must be carried out at high temperatures while the polymer is in a molten state. Classic devolatilization methods include the following: The first method, known as slit flash evaporation, is a classic approach used for polymers with good melt flowability, such as polystyrene and SMA. It involves allowing the polymer melt to flow through a slit under high temperature and pressure, while the container on the other side of the slit is under high vacuum. Once the polymer melt passes through the slit, it suddenly bursts open under vacuum, releasing the volatile components. The advantage of this process is that it breaks the phase equilibrium of the polymer solution, allowing the volatile components to separate, and simultaneously imparts energy to the volatile components during the explosion, resulting in high efficiency. However, the problems are that the process is only a one-time, intermittent process and cannot be continuous; furthermore, the equipment is large, and the high pressure poses a significant risk. The second method is called the surface renewal method using a disc reactor. This method is suitable for resins like polyester resin where continuous devolatilization is desired. The devolatilization device is a large vacuum device containing a row of several large, rotatable discs, half of which are inserted into the resin melt. The discs rotate continuously, carrying the resin out from the inside and into the vacuum. The volatiles on the resin surface then evaporate. This process is repeated continuously, resulting in continuous surface renewal and evaporation. The advantage of this process is that it can continue until the desired result is achieved. However, the problem is that the equipment is also very large, and since it relies solely on evaporation for devolatilization, the kinetic energy is insufficient, resulting in high energy consumption. In recent years, twin-screw extruders have been increasingly used as devolatilization equipment and have received growing attention because the barrel volume is small, the vacuum system does not need to be large, and the surface renewal of high-viscosity polymer melts is easily achieved under the shearing action of twin screws, thus reducing energy consumption significantly compared to disc reactors. However, the devolatilization principle is actually the same as that of disc reactors, relying on surface renewal and vacuum evaporation. It lacks the power to break the phase equilibrium of the polymer solution and impart explosive energy to the volatiles. Moreover, the length of the screw is limited, making the devolatilization process extremely limited. The basic method is to use meshing blocks or even add reverse threads to reduce the local conveying volume of material, and then use large-lead screw blocks to spread the melt to increase the evaporation area. At the same time, the mutual shearing action of the screws achieves continuous surface renewal to achieve a better devolatilization effect. See patents CN116020338B, CN223520161U, and CN222309389U. However, when faced with the encapsulation of high-viscosity polymer melt, especially when the solvent is highly compatible with the polymer, the solvent and exhaust gas encapsulated by the polymer melt are powerless to break free from the constraint of the high-viscosity polymer melt. Even the highest vacuum level is ineffective because the polymer solution phase is not destroyed by self-evaporation, and the energy and power are far from sufficient. The only solution is to extend the length of the devolatilization section and the number of devolatilization ports. This not only increases the investment in equipment costs and generates high energy consumption, but also easily leads to foaming under vacuum when the high-viscosity polymer melt is encapsulated with a large amount of volatiles, clogging the devolatilization ports. It is also easy for the polymer to degrade due to repeated shearing by the meshing block and the reverse thread. In view of the above, it is necessary to improve the existing twin-screw extruder structure for devouring to meet the current needs of product processing. Summary of the Invention

[0003] To solve the above-mentioned technical problems, the present invention provides a co-rotating and counter-rotating flash devouring twin-screw extruder. The extruder includes a system for creating high pressure and instantaneous release. The system for creating high pressure and instantaneous release includes a flash threaded block element, a small-lead feed threaded block, a medium-lead feed threaded block, a large-lead feed threaded block connected to the flash threaded block element, and a matching large-lead feed element for pressure reduction and devouring, a screw barrel, and a vacuum system.

[0004] As a further supplement to this technical solution, the system for creating high pressure and instantaneous release is provided in multiple sets.

[0005] As a further supplement to this technical solution, the crown thickness of the flash threaded block is 0-20mm, and the distance m between the crown and the slit in the inner wall of the screw barrel is 0.5 to 5mm.

[0006] As a further supplement to this technical solution, looking from the extrusion end to the feeding end, the arrangement order is as follows: flash threaded block element, small lead conveying threaded block, medium lead conveying threaded block and large lead conveying threaded block are arranged in sequence.

[0007] As a further supplement to this technical solution, the small-lead conveying threaded block contains at least one complete lead.

[0008] As a further supplement to this technical solution, the other end of each system that cooperates with the flash threaded block element is a conventional or large-lead conveying element with a notch. It receives the polymer melt and volatiles ejected by the flash threaded block element under high pressure, and spreads and conveys the ejected melt to continue evaporation and devolatilization. At the same time, the volatiles that are flashed out of the melt are discharged from the system through the devolatilization port under the action of the vacuum system.

[0009] As a further supplement to this technical solution, a vacuum evaporation port is provided on the screw barrel where the large lead threaded block is located. This vacuum evaporation port is connected to the vacuum system, and its vacuum degree is required to be below 5000Pa. Each vacuum system must be independent of each other.

[0010] As a further supplement to this technical solution, the distance between the connection position of the flash threaded block and the large lead conveying element and the vacuum devaporization port is required to be controlled within the range of 0.5 to 1 lead.

[0011] Its beneficial effects are that the present invention makes full use of the flash evaporation principle, firstly breaking the original phase equilibrium of the polymer solution to separate it, and at the same time transferring the high energy released instantaneously under high pressure to the volatiles, enabling them to break through the constraints of the high viscosity polymer melt and burst out. It not only retains the surface renewal and vacuum devolatilization characteristics of conventional twin-screw devolatilization extruders, but also avoids the drawbacks of easy blockage of the devolatilization port and shearing and degradation of polymer molecular chains. Attached Figure Description

[0012] Figure 1 This is a schematic diagram of a twin-screw extruder for co-directional and anti-directional flash devouring according to the present invention; Figure 2 This is a schematic diagram of the flash threaded block component structure of the present invention; Figure 3 This is a diagram of the conventional twin-screw devouring extruder screw assembly system of the present invention. Detailed Implementation

[0013] To facilitate a clearer understanding of this technical solution for those skilled in the art, the following will be described in conjunction with the appendix. Figure 1-3 The technical solution of the present invention is described in detail below: like Figure 1As shown, a co-rotating and counter-rotating flash devolatilization twin-screw extruder includes a system for creating high pressure and instantaneous release. This system comprises a flash screw block element, a small-lead conveyor screw block, a medium-lead conveyor screw block, a large-lead conveyor screw block connected to the flash screw block element, and a matching large-lead conveyor element for pressure-reducing devolatilization, a barrel, and a vacuum system. The extruder contains at least one set of high-pressure and vacuum release systems, the number of which varies depending on the solvent and other volatile content in the polymer melt and the permissible residual volatile content in the final material. The greater the amount of devolatilization required and the lower the permissible residual volatile content in the final material, the more high-pressure and vacuum release systems are needed.

[0014] The structure of the flash threaded block element is as follows: Figure 2 As shown in the figure cr is the diameter of the threaded block tooth crown of the extruder. The diameter of the tooth root of the rt thread block; angle α is 0. Up to 70 The tooth crown thickness b ranges from 0 to 20 mm, and the distance m from the tooth crown to the slit in the inner wall of the screw barrel ranges from 0.5 to 5 mm; the thread block thickness a depends on the maximum pressure required for devolatilization and the strength and stiffness of the material itself. From Figure 1 As can be seen, in the extruder barrel, the polymer melt is extruded and pushed through the large-lead, medium-lead, and small-lead screw elements, with the pushing volume gradually decreasing, but the pushing pressure gradually increasing. However, when the melt flows to the flash element, it is completely blocked, with almost no passage, so the melt pressure becomes extremely high instantaneously. See [reference needed]. Figure 2 At this point, the only pathway for the melt is the narrow slit m remaining between the outer diameter of the flash threaded block element's tooth crown and the inner wall of the screw barrel. Therefore, the melt is ejected from the slit m, completing the flash evaporation process. (See [reference needed]). Figure 1 Therefore, the smaller the slit size *m*, the greater the pressure of the melt volume. Furthermore, the narrower the tooth thickness *b*, the faster the high pressure is released, resulting in a more complete flash vaporization effect and better devolatilization. However, the size of the slit *m* and the tooth thickness *b* depends on the extruder's machining precision and the strength and rigidity of the material used.

[0015] like Figure 1As shown, looking from the extrusion end to the feeding end, the arrangement sequence is flash threaded block elements, small-lead conveying threaded blocks, medium-lead conveying threaded blocks, and large-lead conveying threaded blocks arranged in sequence. The purpose of this sequential arrangement is to generate high pressure when melt transport is obstructed, laying the foundation for flashing. Therefore, the small-lead threaded block must contain at least one complete lead. The greater the volatility, the lower the allowable residual volatile content in the final material, the longer the structure of this conventional threaded block element combination, and the greater the number of threaded block elements. Of course, their final quantity must also meet the design requirements of the extruder's structure.

[0016] In each system, the other end of the flash threaded block element is a conventional or notched large-lead conveyor element. This element receives the polymer melt and volatiles ejected from the flash threaded block element under high pressure, while simultaneously spreading and conveying the ejected melt for further evaporation and devolatilization. At the same time, the volatiles extracted from the melt through flash evaporation are discharged from the system through the devolatilization port under the action of a vacuum system. The large-lead threaded block is located on a screw barrel equipped with a vacuum devolatilization port, which is connected to a vacuum system with a required vacuum level below 5000 Pa. This function not only aligns with the surface renewal feature of traditional devolatilization extruders but also adds the function of receiving the flash-ejected polymer melt and volatiles. Therefore, the distance between the connection point of the flash threaded block and the large-lead conveyor threaded block and the vacuum devolatilization port must be controlled within the range of 0.5 to 1 lead. This requirement is in place for two reasons. First, it prevents the melt ejected during flash evaporation from entering the vacuum system along with the volatiles if it is directly sprayed into the vacuum evaporation port. Second, it also prevents the volatiles ejected during flash evaporation from having to pass through too many threaded blocks before reaching the vacuum evaporation port, thus reducing the evaporation efficiency.

[0017] Example 1, using one A 95-type co-rotating close-meshing flash devouring twin-screw extruder is used to devour a POE polymer solution containing 75% n-hexane, with a planned output of 100 kg / h. Its screw block configuration is similar to... Figure 1 Similarly, the structure of the flash threaded block is as follows: Figure 2 As shown. Where angle α is 52°. The tooth crown thickness *b* is 3 mm, and the distance *m* between the tooth crown and the slit in the inner wall of the screw barrel is 1 mm. There are a total of 3 flash-evaporation screw blocks, corresponding to the 5th, 8th, and 10th vacuum devouring sections of the screw barrel, respectively. The vacuum levels of the 5th, 8th, and 10th vacuum devouring sections are 26 kPa, 6000 Pa, and 1000 Pa, respectively, belonging to three independent vacuum systems. The devouring capacities of the 5th, 8th, and 10th vacuum devouring sections reach 400 kg / h, 100 kg / h, and 20 kg / h, respectively. The screw barrel temperature reaches a maximum of 120℃ up to the 6th section, a maximum of 150℃ up to the 9th section, and a maximum of 170℃ up to the 11th section. The die temperature reaches 185℃. The devouring results are shown in Table 1.

[0018] Comparative Example 1 uses a machine with the exact same construction as Example 1. The 95 co-rotating close-meshing twin-screw devouring extruders, both devouring POE polymer solutions containing 75% n-hexane, have a planned output of 100 kg / h. The screw block configuration is as follows: Figure 3 As shown, this is a classic devourer screw block combination. The vacuum devouring sections are also located in sections 5, 8, and 10, with vacuum levels of 26 kPa, 6000 Pa, and 1000 Pa respectively, belonging to three independent vacuum systems. The vacuum devouring capacities of each section are 400 kg / h, 100 kg / h, and 20 kg / h respectively. Furthermore, the screw barrel temperature reaches a maximum of 120°C before section 6, 150°C before section 9, and 170°C before section 11, while the die temperature reaches 185°C. The devouring results are shown in Table 1.

[0019] Depend on Figure 3 As can be seen, the principle of devolatilization in a conventional twin-screw devolatilization extruder is that a large-lead threaded block is followed by a small-lead threaded block, and then a set of kneading blocks. Since the kneading blocks have almost no capacity to transport the melt, but have excellent shearing and dispersing effects, this combination of threaded block systems results in a gradual decrease in melt flow rate and a gradual increase in melt pressure. Beyond the kneading blocks, the large-lead threaded blocks are exposed to vacuum, causing the relatively low-flow melt to suddenly enter a high-speed transport zone. This allows the volatiles to evaporate under vacuum on the thinly spread threaded elements, and the surface is constantly renewed with the screw rotation, but it lacks a significant flash evaporation effect. Furthermore, before the ninth stage vacuum devolatilization port, after the kneading blocks, a counter-threaded block is added to strengthen the sealing of the melt, ensuring that the tenth stage can achieve the highest vacuum level.

[0020] Example 2, using one A co-rotating, close-meshing flash devolatilization twin-screw extruder (model 95) was used to devolatilize an SMA solution containing 50% styrene monomer, with a planned output of 140 kg / h. The twin-screw extruder and screw block configuration were the same as in Example 1. The barrel temperature reached a maximum of 140°C up to stage 6, 170°C up to stage 9, and 180°C up to stage 11. The die temperature reached 190°C. The devolatilization results are shown in Table 1.

[0021] Comparative Example 2, using one A co-rotating, tightly meshing twin-screw devolatilization extruder (model 95) was used. The devolatilization target was the same as in Example 2: a solution containing 50% styrene monomer (SMA). The planned output was 140 kg / h. The twin-screw extruder and screw block configuration were the same as in Comparative Example 1. Similar to Example 2, the barrel temperature reached a maximum of 140°C up to stage 6, a maximum of 170°C up to stage 9, and a maximum of 180°C up to stage 11. The die temperature reached 190°C. The devolatilization results are shown in Table 1.

[0022] Example 3, using one A counter-rotating close-mesh flash devouring twin-screw extruder (model 95) was used to devour an SBS solution containing 80% cyclohexane, with a planned output of 90 kg / h. Since SBS is a thermoplastic elastomer, a counter-rotating close-mesh flash devouring twin-screw extruder was employed to increase the driving force. However, the extruder barrel and threaded block configuration were the same as in Example 1. The barrel temperature reached a maximum of 120°C before stage 6, a maximum of 150°C before stage 9, and a maximum of 170°C before stage 11. The die temperature reached 185°C. The devouring results are shown in Table 1.

[0023] Comparative Example 3, using one The 95-type counter-meshing twin-screw devouring extruder was used, with the same devouring material as in Example 3: an SBS solution containing 80% cyclohexane. The planned output was 90 kg / h. Again, since SBS is a rubber, a counter-meshing flash devouring twin-screw extruder was used to increase the driving force. However, the extruder barrel and screw block configuration were the same as in Comparative Example 1. Similar to Example 3, the barrel temperature reached a maximum of 120°C before the 6th stage, a maximum of 150°C before the 9th stage, and a maximum of 170°C before the 11th stage. The die temperature reached 185°C. The devouring results are shown in Table 1.

[0024] Table 1 Comparison results between the embodiments and comparative examples As shown in Table 1, regardless of whether a co-rotating or counter-rotating twin-screw extruder is used for devolatilization of the polymer melt, the devolatilization efficiency of the flash devolatilization extruder of this invention far exceeds that of conventional devolatilization extruders, reaching nearly two orders of magnitude. The reason for this is that after most of the volatile matter is removed, the viscosity of the melt increases dramatically, making conventional devolatilization methods relying on vacuum evaporation ineffective. In particular, the vacuum pipe of the 10th barrel in the vacuum devolatilization section often has very low temperature, indicating that very little volatile matter is removed, yet a significant amount of volatile matter remains in the polymer melt. Furthermore, the shearing effect of the kneading blocks also leads to a decrease in the polymer molecular weight.

[0025] The above technical solutions only embody the preferred technical solutions of the present invention. Any modifications that may be made by those skilled in the art to certain parts thereof embody the principles of the present invention and fall within the protection scope of the present invention.

Claims

1. A twin-screw extruder for co-rotating and counter-rotating flash devouring, characterized in that, The extruder includes a system for creating high pressure and instantaneous release, which includes a flash threaded block element, a small-lead feed threaded block, a medium-lead feed threaded block, a large-lead feed threaded block connected to the flash threaded block element, and a matching large-lead feed element for pressure reduction and devouring, a barrel, and a vacuum system.

2. The co-rotating and counter-rotating flash devolatilization twin-screw extruder according to claim 1, characterized in that, The system for creating high pressure and instantaneous release is provided in multiple sets.

3. The co-rotating and counter-rotating flash devolatilization twin-screw extruder according to claim 2, characterized in that, The thickness of the tooth crown of the flash threaded block is 0-20mm, and the distance m between the tooth crown and the slit in the inner wall of the screw barrel is 0.5 to 5mm.

4. A twin-screw extruder for co-rotating and counter-rotating flash devolatilization according to claim 3, characterized in that, Looking from the extrusion end to the feeding end, the arrangement order is flash threaded block element, small lead conveying threaded block, medium lead conveying threaded block and large lead conveying threaded block arranged in sequence.

5. A twin-screw extruder for co-rotating and counter-rotating flash devolatilization according to claim 4, characterized in that, The small-lead feed threaded block contains at least one complete lead.

6. A twin-screw extruder for co-rotating and counter-rotating flash devouring as described in claim 5, characterized in that, The other end of each system that cooperates with the flash threaded block element is a conventional or large-lead conveying element with a notch. It receives the polymer melt and volatiles ejected by the flash threaded block element under high pressure, and spreads and conveys the ejected melt to continue evaporation and devolatilization. At the same time, the volatiles that are flashed out of the melt are discharged from the system through the devolatilization port under the action of the vacuum system.

7. A twin-screw extruder for co-rotating and counter-rotating flash devouring as described in claim 6, characterized in that, A vacuum evaporation port is provided on the screw barrel where the large lead threaded block is located. This vacuum evaporation port is connected to the vacuum system, and its vacuum degree is required to be below 5000Pa. Each vacuum system must be independent of each other.

8. A twin-screw extruder for co-rotating and counter-rotating flash devouring according to claim 7, characterized in that, The distance between the connection position of the flash threaded block and the large lead conveying element and the vacuum devaporization port is required to be controlled within the range of 0.5 to 1 lead.

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