Method and apparatus for purifying gases from a vacuum zone of a plasticising device

Abstract

The invention relates to a method and to an apparatus for purifying gases which escape from a plastic melt (13) in a vacuum zone (2) of a twin-screw extruder (1) and contain water vapour, air, and hydrocarbon compounds, which gases (14) are removed from the vacuum zone (2) and fed to a resublimator (3), wherein, in the resublimator (3), substances that can condense, can freeze, can resublimate, and / or that are particulate, are at least in part deposited as or in ice by means of a cooling device (4) on the surface thereof. The resublimator (3) is at least in part flooded with hot liquid (11) in such a way that ice formed by the cooling device (4) is melted away and melted down by the cooling device (4), and the melted ice is drained from the resublimator (3) together with the liquid (12).

Description

[0001] Method and device for cleaning gases from a vacuum zone of a plasticizing device

[0002] The invention relates to a method and a device for cleaning gases taken from a vacuum zone of a plasticizing device.

[0003] In principle, it is known to dry gases or granular materials, in particular, by applying a vacuum. For example, EP 1 262 727 B1 describes a conveying and drying system for bulk materials in which dusty or granular material is conveyed under suction or pressure. This avoids the drying of moist, dusty, or granular materials with air heated by electric heaters.

[0004] DE 102007 056610 B4 describes a process for extruding plastic parts in which gases from polymer melts are resublimated on a cooled plate, allowing organic, resublimable substances to be removed using compressed air. A solids separator is also provided for separating oligomers, for example, and vaporous components are removed by condensation. The resublimate is deposited on a plate, from which it can be cleaned using a cleaning device.

[0005] DE 10 2013 000 316 A1 also describes a device for degassing polymer melts, in which the cooling plates used in DE 102007 056610 B4 for resublimating substances to be cleaned are replaced by tubes. The compressed air cleaning described therein is to be replaced by a mechanical cleaning device in the form of a scraper. The cleaning device is intended to remove solid, liquid, and / or pasty components from the cooling tubes. Cleaning is carried out using a scraper by scraping the surfaces of the cooling tubes. Separation takes place in the vacuum separator by resublimation or condensation. To prevent condensation and / or resublimation of the exhaust gases in the pipes, they are heated to a temperature of 150°C to 300°C outside the vacuum chamber.

[0006] JPH 06 190 897 A describes a device for treating exhaust gas from the vacuum zone of a plasticizing device. The exhaust gas is cooled with water flowing in a double-walled housing, so that any ice resublimating during cooling in the vacuum chamber is indirectly exposed to the heat transfer medium flowing in the cooling or heating device.

[0007] DE 196 53613 A1 describes a method and device for removing condensable, solidifiable components from warm exhaust gas streams. The resulting ice melts indirectly via a heat exchanger containing a heat transfer medium at temperatures of 60°C to 80°C. Such indirect melting is disadvantageous, among other things, because a relatively long period of time is required for the ice to melt.

[0008] DE 102014 016 380 A1 provides another possibility for removing pollutants from exhaust gases by treating the exhaust gases in a plasma chamber, in particular a microwave plasma chamber, so that deposits from the substances contained in the exhaust gases can be avoided.

[0009] EP 2 209604 B1 describes a process for extruding plastic parts, in which a resublimate formed in a vacuum chamber is removed from the heat transfer surfaces either with compressed air or with a mechanical cleaning device. This involves a two-stage cleaning of the exhaust gas in the resublimator and a downstream filter, with the resublimator being cleaned either with compressed air or by means of a mechanical cleaning device.

[0010] Furthermore, US 4,353,222 describes a vacuum chamber for freeze-drying processes, particularly for food products in smaller chemical plants. This vacuum chamber contains a steam condenser. Within the vacuum chamber, the exhaust gas is purified by resublimation, and the ice that has formed is indirectly exposed to energy from a heat transfer medium via a heat exchanger to melt and defrost the ice.

[0011] US 3,381,746 describes a steam condenser which defrosts even under atmospheric pressure. During operation, water vapor contained in the air is resublimated to ice, a process used for freeze-drying food and pharmaceutical products. The ice resublimated on the condenser surfaces can, in principle, be melted and removed from the surface of the condenser unit using hot water or steam, without the need for special precautions, as it is only water; gaseous hydrocarbons are not present, which can also be ruled out from the outset when used on food and pharmaceutical products. However, the melting process of the ice formed on the condenser is relatively lengthy due to the internal compactness and the internal surfaces of the condenser.

[0012] Finally, DE 102020 116 414 B3 describes a method and device for purifying gases from polymer melts. In the known method and device, the gas to be purified is taken from a vacuum zone of a plasticizing unit and passed into a vacuum separator for separating condensable, freezable, and / or resublimable substances. The ice formed on the surfaces of the cooling arrangement arranged in the vacuum separator is melted using a heating device with a heating medium at a temperature of at least 100°C to approximately 160°C. Since the melting process obviously still takes too much time after the ice has been detached from the surfaces of the heat exchanger due to impaired heat transfer, a mechanical device is additionally provided to scrape the ice from the pipes or otherwise remove it mechanically.This makes the entire system more complicated, more expensive and more energy-intensive.

[0013] What almost all of these devices and methods have in common is that the ice that primarily forms during resublimation in a vacuum chamber is melted in a well-known manner using a heat transfer medium inside a heat exchanger on the outside of the heat exchanger surfaces. This is disadvantageous, however, because the melting process is relatively long. After the ice melts from the surfaces of the heat exchanger, it detaches from there, and the further melting process only takes place through heat transfer via the air gap between the surface of the heating tube of the heat exchanger. The air gap acts as an insulator to a certain extent, since the heat transfer coefficient from metal to air is lower than, for example, that from the metal of the heat exchanger to ice directly.

[0014] In contrast to pure freeze-drying for food, it is known in the relevant prior art for twin-screw extruders that, in order to purify gases extracted from a vacuum zone of a plasticizing device or plastic melt, i.e., in the production of polyester melts with twin-screw extruders, a vacuum must be applied to the extruder to extract the gases generated during the melting process of the plastic. These gases from the vacuum zone of the plastic melt are essentially composed of water vapor, air, and volatile components from the melt.

[0015] Depending on the moisture content of the granules used, water vapor accounts for 50 to 90% of the volume. For example, with a typical moisture content of 2500 ppm in the granules, approximately 75% of the volume of the extracted gas can be assumed. At a maximum conceivable moisture content of 4000 ppm, the volume fraction is then approximately 85%. Including surface moisture, the volume fraction in the extracted gas can even be as high as 90%. If the water vapor cannot be reliably extracted, the water reacts with the polyester or polymer melt and destroys molecular chains. However, this renders the melt unusable for further film production.

[0016] In addition to the steam, the mixture also contains air, a small part of which is transported along with the granules and melt. This air is also extracted, although in a well-functioning process, this accounts for approximately 5% to 10% of the volume flow. If the air is not extracted reliably, bubbles can form in the finished product, which impairs the quality of the film produced and no longer meets the highest standards. In biaxial film stretching systems, bubbles in the cast film can even lead to a complete production stoppage and damage to the intact casting roll surface, which is crucial for good film quality, due to electrostatic discharges.

[0017] The volatile components also present in the gases and which must be separated from the melt, such as oligomers and / or monomers, also outgas in the vacuum. Their amount depends on the quality of the granulate used, and accordingly, more or less impurities or low-boiling hydrocarbon compounds are ultimately contained in the melt. In a well-functioning process, this can be assumed to amount to approximately 1% to 5% of the gas volume. If such impurities and more volatile components are not reliably extracted, they lead to quality problems in the downstream process of film production. In principle, the volume to be extracted determines the required size, power consumption and thus also the cost of the vacuum pumping station. On the one hand, the very large proportions of water vapor and the volume of air to be extracted are responsible for this.The extracted contaminants and the proportion of these that pass through the separation system have a decisive influence on the possible selection of vacuum pumps and their service life and operational reliability. On the other hand, the required vacuum on the twin-screw extruder is crucial for the size of the volume to be extracted. For polyester extrusions, 3 mbar to 15 mbar are typical values. For biaxially stretched films, the pressure or the required vacuum on the twin-screw extruder is in the order of 5 mbar, because experience has shown that values ​​above 10 mbar can lead to quality and production problems. If these vacuum pressures are not achieved, changes in the mechanical film properties and even increased film or foil tears occur.Since the film stretching system must be stopped, cleaned, and restarted after a film break, a film break significantly negatively impacts the system's productivity. Longer downtimes, in particular, can significantly reduce a system's profitability.

[0018] The condensation or freezing of the water vapor component is crucial for the size of the vacuum pumping station. If a system is designed without water vapor extraction, it goes without saying that the size of the pumping station with water vapor extraction only needs to be approximately 25% of the comparable size of the system without water vapor extraction. If a design pressure of 5 mbar is assumed on the twin-screw extruder, the moisture can no longer be condensed. This well-known relationship is illustrated by the pressure-temperature diagram for the three states of matter: solid, liquid, and vapor, with the so-called triple point (see Figure 1 for an illustration of the principles of this invention).

[0019] At the specified pressure of 5 mbar, water only exists as gas or ice. This means that the only way to reduce the volume is to freeze out the water content in the gas taken from the vacuum zone. It must be taken into account that, in addition to the suction and freezing, there is also a certain pressure loss. The design assumes a recommended pressure loss between the twin-screw extruder and the outlet of the freezing unit of around 1.5 mbar. Taking into account the saturated steam curve shown in Figure 2, a temperature of -7°C is required for freezing out at a process pressure of 5 mbar on the twin-screw extruder and a temperature of -18°C for a process pressure of 3 mbar on the twin-screw extruder. In order to utilize the significant volume reduction described above, the water content must therefore be frozen out.This is achieved through the contact of the gases extracted from the vacuum zone of the plasticizing unit with the cooling unit. This causes a layer of ice to form on the cooling unit from the relatively small mass of water vapor. Relatively little power is required for this small mass of water vapor (approximately 20 kW for 7,000 kg / h of PET). Due to this relatively low power, the poorer thermal conductivity of ice (2.21 W / mK) compared to that of stainless steel (15 W / mK) plays only a minor role in the cooling unit. Therefore, even with increasing icing, operation can continue for a relatively long time. However, if the pressure loss in the resublimator becomes too great due to the narrowing of the gas flow paths as a result of the increasing thickness of the ice layers, the resublimator must be cleared of ice.

[0020] According to the state of the art, this is achieved by using the cooling device as a heat exchanger by passing a higher-energy heat transfer medium through it. First, the resublimator must be shut down for defrosting, so that the melting phase takes place with appropriate ventilation of the resublimator. This is necessary to ensure that the frozen water does not evaporate again, but can be removed from the resublimator as a liquid at normal pressure. The melting of the ice or ice layer occurs precisely because the cooling device acts as a heating device, supplying the energy required to melt the ice.

[0021] The decisive disadvantage of this prior art solution for melting ice in the resublimator is that, as melting begins, the heat transfer conditions from the heating tube to the ice thermodynamically deteriorate. When the layer of ice adjacent to the cooling tubes of the cooling device thaws, causing the ice to lose contact with the actual cooling device, the water between the ice and the surface of the cooling device flows away, forming air gaps. This means, however, that the heat from the interior of the cooling device—that is, from the surface of the cooling device facing the ice—must be transferred across the air gap created by defrosting. As is well known, air has a thermal conductivity of 0.0262 W / mK, which means that the heat from the cooling device used as a heating device is transferred to the ice 100 times more slowly through the air gap.This involves a very long defrosting process, the duration of which cannot be reliably calculated and may require the entire system to be shut down, significantly detrimentally impacting the effectiveness and profitability of such a system. Since the air gaps can vary in size, and pieces of ice can fall from the cooling device into the vacuum system of the plasticizing device and then be surrounded by air, it takes even longer for this loose ice to melt in the resublimator. However, melting is necessary because the falling ice pieces, in particular, can block the drain and trigger an alarm because the partially melted ice prevents the water from draining away.In such a case, manual intervention is required, meaning the resublimator must be cleaned manually via a maintenance opening. Without such thorough cleaning, which first requires ice removal, the two resublimators belonging to the entire system cannot be used, resulting in a production stoppage.

[0022] The present invention aims to eliminate this crucial disadvantage.

[0023] The object of the invention is therefore to reliably melt the ice formed in a resublimator or vacuum chamber in a melting time that is significantly shorter than the prior art. This increases the operational time of a resublimator when cleaning gases generated and extracted in a vacuum zone of a twin-screw extruder of a plastic melt. This also reduces the risk of ice particles remaining in the resublimator and potentially causing blockages in the resublimator outlet to virtually zero. Liquid remaining in the sublimator would have a very detrimental effect on the evacuation time because this liquid would evaporate again as the pressure drops, placing a significant strain on the pumping station.

[0024] This object is achieved by a method having the features according to claim 1 and by a device having the features according to claim 7.

[0025] Further developments of the invention are defined in the respective dependent claims. According to a first aspect of the invention, a method is provided for purifying gases that contain water vapor, air, and hydrocarbon compounds present in a twin-screw extruder vacuum zone of a plastic melt. These gases are removed from the vacuum zone and fed to a resublimator. In the resublimator or the vacuum device, condensable, freezeable, resublimable, and / or particulate substances are at least partially separated from the gases as or in ice by means of a cooling device on the surface thereof.According to the invention, the above-mentioned object with regard to the method is achieved in that the resublimator is at least partially flooded with hot liquid in such a way that the ice formed by the cooling device is melted onto and off the cooling device and the melted ice is discharged from the resublimator together with the liquid.

[0026] In the context of this invention, flooding means that hot liquid is fed into the interior of the resublimator in such a quantity or with such a flow rate that the resublimator is filled at least to the extent that the ice to be melted is immersed in the hot liquid.

[0027] By flooding the resublimator with hot liquid, the ice is melted. Air gaps between the ice resublimated from the gases and the surface of the cooling device no longer play a disadvantageous role because these gaps are filled with the hot liquid surrounding the ice in every cavity. The molten ice dissolves in the hot liquid, causing its temperature to drop. Heat transfer, i.e. the direct heat transfer from the hot liquid to the ice without air gaps and thus without delay, is always reliably maintained to its full extent due to the direct contact of the hot liquid with the ice. This reduces the melting process from several hours in a state-of-the-art process, which usually also leaves a residual amount of ice and can clog drain lines, to just a few minutes.By selecting the correct ratio of ice to hot liquid, the thawing time should ideally not exceed 5 to 10 minutes in a realistic design. Increasing the amount of hot liquid in the resublimator as a safety margin—i.e., increasing it slightly above the calculated amount—can prevent the risk of residual ice from occurring in the first place. If necessary, the resublimator could be filled or flooded with hot water several more times after the melting phase to improve the cleaning effect. Flooding and the direct contact of hot liquid with the ice in the resublimator can increase its operating time from approximately 70%, as is typically the case with state-of-the-art technology, to approximately 90%. This can also increase profitability.On the other hand, operational reliability can be increased enormously because the ice in the resublimator is completely melted without any residual ice and can be easily removed from the resublimator without clogging the drain line with ice pieces.

[0028] Preferably, the hot liquid also cleans the cooling device on its surface without any mechanical cleaning device.

[0029] While it is possible to add a cleaning agent to the hot liquid to more effectively, easily, and reliably remove any contaminants adhering to the surface of the cooling device in the resublimator and to remove them when the melted ice is drained away, this is not absolutely necessary, as these components are generally resublimated with the ice and are contained in the ice, so that when the ice melts and the melted ice is drained from the resublimator in the form of water, the contaminants are also removed. This represents a significant advantage over prior art devices that use a mechanical cleaning device, often in the form of a scraper, to accelerate ice removal. According to the invention, mechanical cleaning of the cooling device can be dispensed with.Compared to condensation into a liquid phase, resublimation into the ice phase also has the advantage that the ice forms a protective layer around the refrigeration system, preventing any further resublimated contaminants from coming into contact with the surface of the refrigeration system but instead being frozen in the ice. They are then pumped out along with the melted ice. Since the resublimator is easier to clean than a state-of-the-art one, the former is also simpler in design, thus reducing system costs.

[0030] Further preferably, according to the method according to the invention, the cooling device is already switched to its deep-freeze mode during the emptying of the resublimator flooded with the hot liquid, which is followed by the evacuation of the resublimator after emptying. If the cooling device has already resumed deep-freezing even though the resublimator is not yet completely empty, operating time for the resublimator can still be gained because its emptying and the deep-freezing of the cooling device take place at least partially in parallel. When the resublimator is then evacuated, the cooling device is already in deep-freeze mode, so that the resublimator can promptly initiate the process of resublimating the gases. Therefore, the cooling device can preferably be designed exclusively for cooling, thus eliminating the dual use common in the prior art of using the cooling device for both cooling and heating.This also allows for a simpler and more cost-effective design, because, for example, the cooling system only needs to be connected to a liquid circuit containing refrigerant. Downtime is eliminated, especially if the cooling system and the emptying of the resublimator run in parallel, at least for a time.

[0031] The cooling device is preferably operated with either -7°C or -18°C cold liquid, so it does not have to withstand a wide temperature range, for example between -7°C and +90°C, as is the case with a dual-purpose cooling device, i.e. a cooling device that is used for both cooling and heating, as is common in the prior art. Because the coolant can circulate again as soon as the resublimator is emptied, the risk of the cooling device, also called a chiller, receiving water that is too hot and emitting a fault signal if it were to no longer provide the cooling capacity under such operating conditions can be eliminated. This also contributes to increasing the operational reliability of the resublimators. The resublimators are preferably evacuated to 3 mbar and cooled to -18°C or evacuated to 5 mbar and cooled to -7°C.

[0032] Furthermore, water is preferably used as the hot liquid. This water is heated or stored in a tank filled with hot liquid and then fed into the resublimator at the temperature required to melt the ice. The hot liquid can also be glycol, for example. It must be able to introduce the amount of heat into the resublimator that is required to melt the maximum amount of ice that can be produced and can therefore heat the resublimator accordingly. The melting energy of 335 kJ / kg of ice to water corresponds to the energy released by 1 kg of water when cooling from 80°C to 0°C. In order to have enough hot liquid for melting and to maintain sufficient reserves of hot liquid, the circulating amount of hot liquid is selected to be two to three times the maximum amount of ice that can be produced in the resublimator.For the large film stretching systems in question here, this is about 1 m. 3of hot liquid. The liquid is circulated so that the melted ice then represents the excess and thus the only waste. According to the process, the water consisting of melted ice is fed to the tank so that the solids that were dissolved in the ice or absorbed by it can also be removed from the water. Preferably, a double filter is used for double filtration to separate any remaining particles such as oligomers. However, it is also possible for the impurities absorbed in the meltwater to settle in the tank, so that it is cleaned from time to time. This creates minimal amounts of wastewater or waste with maximum concentration, which is beneficial for the environmental compatibility of the process because only small quantities need to be disposed of and, as already stated, the majority of the water, i.e. the hot liquid, can be circulated.

[0033] According to a second aspect of the invention, a device is provided which extracts gases containing water vapor, air, and hydrocarbon compounds from a plastic melt in a twin-screw extruder vacuum zone in film stretching systems and feeds them to a resublimator for purification. The device has a cooling device integrated into the resublimator, by means of which condensable, freezeable, resublimable, and / or particulate substances can be at least partially deposited on its surface. The resublimator is connected via a hot liquid line to a tank from which the hot liquid is supplied to the resublimator, so that it can be or is flooded with hot liquid.The hot liquid can thus melt the ice resublimated in the resublimator and the impurities contained in the ice can be drained from the cooling device together with the melted ice as a liquid from the resublimator and fed back into the tank.

[0034] Further preferably, the device comprises a filter unit downstream of the resublimator, by means of which impurities can be separated from the melted ice. The purified water from the filter unit can then be fed to a water ring pump station used to generate a vacuum. The liquid cooled by melting the ice can be fed to the tank for hot liquid. Further preferably, the volume of the tank is two to three times the volume of the resublimators.

[0035] Further preferably, the cooling device in the resublimator is designed with a large surface area and can be embodied as a corrugated stainless steel tube. Due to the corrugated surface, the stainless steel corrugated tube has a specific surface area twice as large as that of a smooth tube, such as that used in the prior art. In such a case, mechanical cleaning of the ice from the surface of the cooling device must be possible, which requires a smooth surface. However, according to the present invention, mechanical cleaning is expressly not required, which is why the surface of the tube can also be thermodynamically optimized.Such a corrugated pipe made of stainless steel also has the advantage that it has a self-cleaning effect and that solid deposits such as ice can be blown off by the movement of the flexible corrugated pipe, also as a result of existing temperature differences, so that in addition to flooding the resublimator, easier melting and subsequent melting of the melted ice can be achieved.

[0036] Preferably, a known filter unit is arranged downstream of the resublimator, in which filter unit solids absorbed or formed in the gas are filtered out. Preferably, the resublimator, together with this downstream filter, forms the cleaning system in the form of a structural unit; in particular, the filter and cooling device of the structural unit are preferably designed as a common interchangeable insert. Once the solids have been filtered out of the gas, the lines to the pumping station remain clean, thereby increasing the service life of the pumps. A large portion of the previously described oligomers are filtered out in these filters. The cleaner the remaining gas, the more options are available for the downstream pumping station. Ideally, this gas upstream of the pumping station consists only of the second component described above, namely air, which can be discharged into the environment downstream of the pumping station.

[0037] Preferably, the resublimator has a double-walled housing through which the cooling medium can flow, so that by means of the cooling medium, the resublimator can be deep-frozen in conjunction with the flooding of the resublimator with hot liquid after the resublimator has melted down for its renewed cooling function.

[0038] Dry-running vacuum pumping stations generally have two vacuum pumps, with a rotary lobe pump preferably being used as the first vacuum pump. This compresses the gas by a factor of approximately 5. A dry-running screw pump, which can compress the gas to ambient pressure of 1,013 mbar, can be used as the second vacuum pump. Such a dry-running screw pump follows downstream of the first vacuum pump. Such screw pumps are extremely sensitive to particles and contamination due to the small minimum gaps between the rotors and the housing and the relative movement of the gas in the longitudinal direction of the rotors. The gaps between the rotors and the housing of such screw pumps are on the order of 0.05 mm. For this reason, the use of a second rotary lobe pump, which compresses the gas again by a factor of 5, is preferably recommended.A third pump is then provided downstream of the second rotary lobe pump. This third pump is designed as a water ring pump and is suitable for the high operational reliability of such a system. A rotary lobe pump does not allow longitudinal movement of the gas and has gap dimensions that are larger than those of a screw pump. A water ring pump has gap dimensions that are in the range of 1 mm, i.e. 20 times those of a screw pump. With such pumps, particles that have not yet been separated out in the gas virtually never cause damage. Their operating fluids are circulated and cooled between them. Due to the resublimation of the water vapor and the separation of the volatile components, only very few contaminants enter the fluid circuit of the water ring pump, which is why this fluid can be circulated for a very long time before replacement is necessary.

[0039] The disadvantage of such a liquid ring pump, namely the consumption of operating fluid, is reduced by the continuous circulation of the fluid. The small amount of operating fluid still required can be fed into the hot fluid tank to keep the concentration of soluble contaminants low or balanced.

[0040] One problem with vacuum systems with twin-screw extruders for polyester is that acetic acid is produced, which continually lowers the pH value of the circulating liquid. Therefore, the resublimator, cooling system, piping system, pump, and tank for the hot liquid are preferably constructed of acid-resistant material.

[0041] Therefore, the system preferably features pH measurement with additional caustic soda dosing. This makes it possible to adjust the wastewater flow to the same level as with a dry-running vacuum pump. It should be noted that dry-running pumps require a flushing medium to loosen deposits on the rotors and the inner housing wall. This flushing medium is usually diesel or glycol and is not normally required by a water ring pump. However, the two upstream rotary lobe pumps do require such a flushing medium. Therefore, it is also preferably possible to operate both the water ring pump and the circuit through the resublimators with glycol. This can reduce the risk of corrosion, and additional flushing agent is no longer required.This is particularly advantageous if the twin-screw extruder is to be used in a polymerization plant for polyester material, which always has its own glycol treatment plant. This configuration eliminates the generation of liquid waste.

[0042] As previously stated, flooding the resublimators eliminates the need for any mechanical cleaning device at all. This allows for a combination of resublimator and filter unit in a single unit, resulting in a very compact design. This compact design is advantageous given that the space available in the extrusion production area is generally very limited. Furthermore, in such a compact system, the outer ring of suspended filter elements insulates the internal cooling system, which offers energy advantages because no cold is conducted to the outside, making it fully available for the resublimation process; insulation is even unnecessary.

[0043] If the filter and cooling system are designed as a replaceable unit, then if cleaning is required, such a replacement set can be quickly replaced with minimal production interruption, which further increases operational reliability, especially since inserting the filters would otherwise have to be done very carefully and would be correspondingly time-consuming. If the filter bags are not inserted correctly and precisely, or if they do not seal properly, particles cannot be filtered out, which can lead to problems at the pumping station, which can be particularly true when using dry-running pumps.

[0044] Preferably, the device comprises two resublimators and two filter units, which can be operated in pairs as a resublimator and downstream filter unit, which can also be referred to as a separator, individually or in parallel, so that in the case of cleaning resublimated ice, at least one pair and thus the device as such can continue to be operated.

[0045] According to a further preferred embodiment, the resublimator is flooded with the hot liquid from the tank via the line by means of a first pump. The provision of a first pump for flooding the resublimator from the tank in which the hot liquid is stored makes the arrangement of the tank independent of the system.

[0046] Preferably, a further pump, a second pump, is provided, which removes the liquid, which is produced in the resublimator by the melting and melting of the ice and the associated dissolution of the impurities in the resublimator, from the resublimator and thus returns it to the tank.

[0047] Further advantages, details, and possible applications of the present invention will now be explained in detail using an exemplary embodiment with reference to the accompanying drawings. The drawings show:

[0048] Figure 1 : the pressure-temperature diagram with triple point for water;

[0049] Figure 2: the pressure-temperature diagram with triple point as saturation vapor pressure curve for the phase transition areas steam solid and vapor liquid;

[0050] Figure 3: a sectional view of a resublimator according to the invention; and

[0051] Figure 4: A schematic diagram of a device or method according to the invention for purifying gases from the vacuum zone of a twin-screw extruder for film stretching systems. Figures 1 and 2 serve to provide a better understanding and form the basis for the thermodynamic-physical conditions that occur in the resublimator 3 for purifying gases extracted from the vacuum zone of twin-screw extruders 1, and how the corresponding units must be designed so that the system or method can operate according to the invention.

[0052] Figure 1 shows the pressure-temperature diagram with triple point 21 and critical point 27 to illustrate the phase transition for the respective pressure and temperature ranges within which the corresponding states of matter or phases, water vapor, water, and ice, exist. As is well known, triple point 21 is defined by the temperature of a substance and the pressure to which it is exposed – in this case, water – at which the three states of matter, solid, liquid, and gas, are in a state of equilibrium. The diagram shows that the triple point temperature of water is 0.01°C and the associated pressure is 6 mbar. The phase diagram shows the different states of matter under the conditions of pressure and temperature. Based on the respective conditions, it can be determined at which parameters the transitions between solid and liquid, liquid and gas, or gas and solid take place.The area adjacent to the pressure axis is the area where ice 20 is present. The area adjacent to the temperature axis represents the area of ​​water vapor 18. Finally, the area enclosed on both sides in a roughly triangular shape by ice 20 and water vapor 18 represents the area of ​​water 19.

[0053] This shows that the required pressure of 5 mbar on the twin-screw extruder prevents the moisture contained in the gas escaping from the plastic melt from condensing. At this pressure, water exists only in the form of steam or ice. Therefore, the only possible way to reduce the volume is to freeze the water content from the extracted gas.

[0054] Figure 2 shows the saturation vapor pressure curve 22 for water vapor over ice, indicating the triple point 21, and beyond the triple point 21 at higher temperatures, the saturation vapor pressure curve 23 for water vapor over water. The diagram shows the corresponding calculation formulas for the respective ranges, which lead to the values ​​indicated in the diagram. Since a recommended pressure drop of approximately 1.5 mbar is also desirable between the twin-screw extruder and the outlet of the freezing unit, i.e., the resublimator, the required temperature for freezing is shown in Fig. 2. At a process pressure of 5 mbar on the twin-screw extruder, this is -7°C, and at a process pressure of 3 mbar on the twin-screw extruder, this is -18°C.

[0055] Figure 3 shows a cross-section through a resublimator 3 as the main element or main unit of the present invention, both for the method and for the device. The basic structure of the resublimator 3 belonging to the invention is shown in a sectional view in Figure 3. The resublimator 3 has a housing, inside which a cooling device 4 is arranged. This cooling device has corrugated tubes 15 wound helically around a vertical axis over essentially the length of the cooling device insert, so that a relatively thick packing of these corrugated tubes 15 is arranged inside, through which the gas 14 to be purified is passed via an inlet. A coolant 17 flows inside the corrugated tube 15, by means of which the gas 14 to be purified is cooled to such an extent that, taking into account the vacuum prevailing in the housing of the resublimator 3, condensable, resublimable, freezeable and / or particulate substances are separated.If the conditions regarding pressure and temperature inside the resublimator 3 explained above with reference to Figures 1 and 2 are set or maintained accordingly, the aforementioned substances can settle on the surface of the cooling device 4, i.e., the corrugated tubes 15. The corrugated tubes 15 have a decisive advantage over smooth tubes used in the prior art; they have a significantly larger surface area than smooth tubes, thereby enabling significantly improved heat transfer. However, the corrugated tubes 15 cannot be cleaned with simple scraper-like means, as is the case in the prior art. The corrugated tubes 15 therefore provide the necessary large surface area while maintaining the appropriate compactness of the resublimator 3 in the system. They must also exhibit good acid resistance, be inexpensive, and, if possible, not be custom-made.The corrugated tube 15 can be wound in a holding cage in a cooling device 4 designed as a heat exchanger, so that the gases 14 to be purified flow from the coolant 17 to the outside of the corrugated tubes 15 via the actual heat transfer surface. Their basic structure thus enables good heat exchange. Because the corrugated tubes 15, which are flexible and comparable to hoses, are loosely suspended and exposed to temperature changes from warm to cold and vice versa, they are capable of moving due to the temperature differences inside the cooling device 4 or the resublimator 3, which can cause material condensed or deposited on the surface to flake off again.The chipped material can then be removed from the resublimator 3 with the melted ice via an outlet line (not shown in the section) by means of the liquid 12, which has a lower temperature than the hot liquid 11 due to the melting of the ice, during cleaning of the cooling device 4 in the resublimator 3 and is fed to a tank 8 (see Figure 4) for reuse and recycling. With a resublimator 3 constructed in this way with a cooling device 4 located therein, the significant portion of water vapor and hydrocarbons can be resublimated on the surface of the cooling device 4. The water vapor and hydrocarbons either precipitate directly on the surface, or the hydrocarbons precipitate in the ice formed on the surface.If too much ice has formed, the flow resistance of the gas 14 to be cleaned increases as it flows around the corrugated pipes 15 of the cooling device 4, which form the actual heat exchanger, so that the resublimator 3 must be switched off and adjusted to ambient pressure and hot liquid 11 can be supplied so that the ice is melted at the cooling device 11 and discharged from the resublimator 3 as liquid 12, which has a significantly lower temperature than the hot liquid 11, and fed to the tank 8 (see Figure 4) not shown in Figure 3 for reprocessing.

[0056] Figure 4 shows the basic system diagram of a gas purification system, by means of which gases degassed in a vacuum zone of a twin-screw extruder can be purified. The plastic melt 13 is located in a heated container, from which it is fed for further processing by means of a twin-screw extruder 1. A vacuum is applied to the twin-screw extruder 1 in order to extract gases 14 produced during the melting process. The gas 14 to be purified is withdrawn from the twin-screw extruder 1 with its respective vacuum zone 2, by a water ring pumping station 10 creating the necessary vacuum pressure so that the outgassed gas 14 can be fed via correspondingly heated lines to two resublimators 3 with a cooling device 4 located therein. Heating the lines from the vacuum zone 2 to the resublimator 3 to a temperature of approximately 265°C to 285°C is necessary to prevent condensation orTo prevent the precipitation of oligomers in these lines and to avoid clogging them. A heating jacket 24 is typically used for this purpose. The system contains two resublimators 3 connected in parallel, with a filter unit 9 downstream of each resublimator 3. The filter units 9 are intended to separate any oligomers that have not yet been removed from the gas 14 to be purified, so that as few foreign substances as possible reach the pump of the water ring pumping station 10. Ideally, the purified gas upstream of the water ring pumping station 10 consists only of air, which can be discharged into the environment after the pumping station; this is not shown separately.The parallel dual arrangement of resublimator 3 and filter unit 9 makes it possible to operate the entire system continuously and, if necessary, when icing within the cooling device 4 increases the flow resistance, to shut it off from the circuit and clean it accordingly. This is followed by the restart of the cleaned resublimator 3, and if necessary, the second resublimator 3 arranged in the opposite strand is shut down and then cleaned. For the sake of simplicity of illustration, the line from the vacuum zone 2 of the twin-screw extruder 1, which leads to the respective resublimator 3, is referred to as the gas to be cleaned 14.

[0057] For the purpose of cooling, so that the resublimation process can be carried out during operation in the resublimator 3, a coolant 17 is supplied to the cooling device 4 of the resublimator 3 so that, in accordance with the sublimation conditions in the resublimator 3, the gas to be purified is cooled to the low temperature corresponding to the corresponding vacuum pressure in the resublimator 3.

[0058] If the resublimator 3 is too heavily iced up and needs to be cleaned, it is, so to speak, taken out of the circuit, i.e. the supply of gas 14 to be cleaned to the switched off resublimator s is interrupted, so that the gas 14 is only fed to the other resublimator 3. This is followed by the supply of hot liquid 11 via line 5 into the resublimator s. The hot liquid 5 is fed from the tank 8 via the first pump 6 to the resublimator s to be cleaned. The supply of hot liquid 11 via the designated line 5 into the resublimator s serves to melt the ice formed on the surface of the actual cooling elements of the heat exchanger, which is achieved by flooding the resublimator 3. Flooding significantly accelerates the melting time of the ice compared to indirect melting processes as implemented in the prior art via heat exchangers.Melting via heat exchangers has the disadvantage that the ice deposited on the outer surface of the cooling elements is only melted at its direct point of contact with this surface, so that an air gap forms between the surface of the pipe and the ice that has not yet melted, which insulates and thereby impairs heat transfer. In order for the ice to be melted, energy must be supplied via the heat exchanger for a relatively long time until all of the ice has melted. In contrast, flooding the resublimator 3, whereby according to the invention the melting time is considerably reduced, since the ice to be melted or melted by the cooling units is always in the hot water or in the hot liquid, so that an air gap does not occur and the ice can be melted quite quickly by immersing the ice in hot liquid.After the ice has melted in the resublimator 3, it can be drained off together with all other impurities, such as particulate impurities, that have accumulated in the cooled liquid 12 and fed back to the tank 8 via the second pump 7.

[0059] The system for cleaning gases from vacuum zone 2 of a twin-screw extruder 1 for a film stretching system for producing films can now be operated in such a way that only one strand of the parallel double arrangement of resublimator 3 and filter unit 9 is used. However, it is also possible, in addition to the alternating use of one strand with corresponding cleaning cycles of the resublimator 3 with its associated filter unit 9, which are operated alternately one after the other, to operate both resublimators 3 with their respective downstream filter units 9 in parallel. This allows the overall cleaning throughput of the system to be increased. In this case, the two resublimators are cleaned at longer intervals but in close succession. One resublimator must always be under vacuum so that the film production process can proceed without interruption.A double-strand arrangement also has the advantage that one strand is always ready in case the operating resublimator needs to be cleaned. If only one strand is operated at a time and the other is made operational again as quickly as possible after necessary cleaning, the operating resublimator can be switched off immediately and the standby resublimator put into operation straight away if necessary. The tank 8, from which the hot liquid 11 is fed via the pump 6 through the lines 5 to the respective resublimators 3 to thaw the ice resublimated therein, and into which the liquid 12, at a temperature reduced by the melting of the ice, is fed via the pump 7, is made of stainless steel in this embodiment because the pH of the liquid 12 can be low. This means that caustic soda should be added to neutralize the pH.

[0060] Neutralization makes it possible to reuse the water repeatedly in the cycle. It is also possible for the water from the water ring pumping station 10 to be fed into tank 8. This means that the water ring pumps of the pumping station 10 must be rinsed as needed, i.e., depending on the separation efficiency of the resublimator 3 and the filter unit 9. This water can thus be used twice. Wastewater will then be generated in a relatively small amount. Contaminants from the rinse water 25 of the water ring pumping station 10 and impurities contained in the liquid 12 are drained into tank 8 via an overflow 26, collected, and disposed of. The size of the tank 8 or hot water container should be approximately three times the volume of a resublimator, so that the temperature in the tank 8 does not drop too much when refilling several times. A suitable size for a conventional system is approximately 1 m 3To raise the liquid in tank 8 by 50°C in a relatively short time of 20 minutes, approximately 90 kW is required. If approximately 350 l are to be pumped out in 5 minutes, the pump should have a capacity of 4.2 m 3 / h. The melting energy will be approximately 2.6 kW, allowing the melted ice to flow into an overflow storage tank and then only need to be heated to the required heating temperature of 90°C. Combined with the heating of the steel material, approximately 4 kW will be required. It turns out that the energy consumption is very moderate because only a relatively small amount of water is separated.

[0061] The mentioned hot water tank or tank 8 with a size of 1 m 3 It therefore has a heat exchanger with a capacity of approximately 90 kW, an overflow, insulation, a fresh water connection and possibly a device for measuring the pH value.

[0062] The use of water ring pumps for the water ring pumping station 10 has the advantage that they can better cope with non-cleaned particulate matter in the gases 14 to be cleaned. Their operating fluid must sometimes be replaced at specified intervals. The method and device according to the invention make it possible to significantly reduce the downtime of the resublimator 3 for the purpose of thawing the ice in the cooling device 4. For a system according to the prior art, approximately 150 minutes are required for ventilation, heating, holding, pre-cooling, evacuation, and deep freezing, with an operating time of 360 minutes, during which the system or the respective resublimator 3 is out of operation; this results in an operating time of only 71%.

[0063] According to the present invention, only 50 minutes are required for venting, flooding, holding, emptying, freezing, and evacuating the resublimator 3, assuming an operating time of 360 minutes. This corresponds to an operating time of 88%. This clearly shows that the operating time of the systems can be significantly increased by melting the ice on the cooling device 4 of the resublimators 3 by flooding them with hot liquid 11.

[0064] List of reference symbols:

[0065] 1 twin-screw extruder

[0066] 2 vacuum zones

[0067] 3 Resublimator

[0068] 4 Cooling device / chiller

[0069] 5 Hot liquid line

[0070] 6 first pump

[0071] 7 second pump

[0072] 8 Tank hot liquid

[0073] 9 Filter unit

[0074] 10 water ring pumping station

[0075] 11 hot liquid

[0076] 12 Liquid

[0077] 13 Plastic melt

[0078] 14 gas to be purified

[0079] 15 flexible corrugated pipe

[0080] 17 Refrigerants

[0081] 18 Water vapor

[0082] 19 Water

[0083] 20 ice creams

[0084] 21 Triple Point

[0085] 22 Saturation vapor pressure curve of water vapor over ice

[0086] 23 Saturation vapor pressure curve of water vapor above water

[0087] 24 heating jackets

[0088] 25 Operating fluid

[0089] 26 Overflow

[0090] 27 critical point

Claims

PATENT CLAIMS 1. A method for purifying gases which escape from a plastic melt (13) in a vacuum zone (2) of a twin-screw extruder (1) and comprise water vapor, air, and hydrocarbon compounds, which gases (14) are taken from the vacuum zone (2) and fed to a resublimator (3), wherein condensable, freezeable, resublimable, and / or particulate substances are deposited at least partially as or in ice in the resublimator (3) by means of a cooling device (4) on the surface thereof, characterized in that the resublimator (3) is at least partially flooded with hot liquid (11) in such a way that the ice formed by the cooling device (4) is melted and remelted by the cooling device (4), and the molten ice is discharged from the resublimator (3) together with the liquid (12).

2. Method according to claim 1, characterized in that the hot liquid (11) cleans the cooling device (4) on its surface without a mechanical cleaning device.

3. Method according to claim 1 or 2, characterized in that a cleaning agent is added to the hot liquid (11), which detaches adhering contaminants from the cooling device (4) and carries them away.

4. Method according to one of claims 1 or 2, characterized in that deep freezing of the cooling device (4) already takes place during the emptying of the flooded resublimator (3), which is followed by its evacuation after its emptying.

5. The method according to claim 4, characterized in that the resublimator (3) is evacuated to at least 3 mbar and cooled to -18°C or evacuated to at least 5 mbar and cooled to -7°C.

6. Method according to one of claims 1 to 5, characterized in that the hot liquid (11) is in particular water or glycol, has a temperature of ^90°C and is led from a tank (8) for flooding the resublimator (3).

7. Device for film stretching systems, with which gases (14) containing water vapor, air and hydrocarbon compounds can be removed from a plastic melt (13) in a vacuum zone (2) of a twin-screw extruder (1) and can be purified by taking them from the vacuum zone (2) and feeding them to a resublimator (3) and in the resublimator (3) by means of a cooling device (4) on the surface of said gases (14) condensable, freezeable, resublimable and / or particulate substances can be at least partially separated from said gases (14), characterized in that the resublimator (3) is flooded via a line (5) with a hot liquid (11) provided from a tank (8) and the ice formed on the surface of the cooling device (4) can be melted and removed,the impurities contained in the ice can be dissolved and then a liquid (12) can be drained off from the resublimator (3) together with the melted ice and the impurities and fed back into the tank (8).

8. Device according to claim 7, characterized in that downstream of the resublimator (3) and a filter unit (9) there is arranged a water ring pumping station (10) for generating vacuum, the rinsing liquid of which can be fed to the tank (8).

9. Device according to claim 7 or 8, characterized in that the volume of the tank (8) corresponds to two to three times the volume of the resublimator (3). 10 Device according to one of claims 7 to 9, characterized in that the hot liquid (11) has a temperature of < 90°C when entering the resublimator (3).

11. Device according to one of claims 7 to 10, characterized in that the hot liquid (11) is glycol or water.

12. Device according to one of claims 7 to 11, characterized in that the resublimator (3) and the filter unit (9) connected downstream thereof form a structural unit.

13. Device according to one of claims 7 to 12, characterized in that The filter unit (9) and cooling device (4) of the assembly are designed as a common interchangeable insert.

14. Device according to one of claims 7 to 13, characterized in that the resublimator (3) has a double-walled housing through which the coolant (17) flows.

15. Device according to one of claims 7 to 14, characterized in that the resublimator (3) is flooded with the hot liquid (11) from the tank (8) via the line (5) by means of a first pump (6).

16. Device according to claim 15, characterized in that the liquid (12) can be fed back to the tank (8) by means of a second pump (7).

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