Apparatus for the underwater pelletizing of plastics by pressurized water at a process-water temperature of over 100°c

Abstract

A description is given of an apparatus for the underwater pelletizing of plastics by pressurized water at a process-water temperature of over 100°C, wherein a pelletizing unit (1), a dehydrating unit (2) for separating the process water from the pellets of plastic and a feed pump (3) are flow-connected to one another by way of a common pressurized-water circuit (4), and wherein a bypass line is provided in the pressurized-water circuit (4) for the pelletizing unit (1). To design such an apparatus in such a way as to allow vacuole-free pellets to be produced in a way that is energy-efficient and saves water overall, even taking into account any downtimes in which it is necessary for the housing of the pelletizer to be opened safely for inspection or cleaning purposes or when changing over batches, it is proposed that provided between the dehydrating unit (2) and the feed pump (3) is a heating pressure vessel (6), to which the pelletizing unit (1) is flow-connected by way of a return line (7).

Description

[0001] Device for pressurized underwater granulation of plastics at a process water temperature of over 100 °C

[0002] Technical area

[0003] The invention relates to a device for pressurized water underwater granulation of plastics at a process water temperature of over 100 °C, wherein a granulation unit, a dehydration unit for separating the process water from the plastic granulate and a feed pump are fluidly connected to one another via a common pressurized water circuit, and wherein a bypass line is provided in the pressurized water circuit for the granulation unit.

[0004] State of the art

[0005] Particularly during the underwater granulation of amorphous plastics with a glass transition temperature above 100 °C, such as polycarbonate, regular care must be taken to ensure that undesirable vacuole formation on the granules, which would impair processing quality, does not occur during the processing. Devices have therefore already been proposed (EP 2361174 A1), in which a granulation unit for cutting a melt strand into granules, a dehydration unit for separating the process water from the granules, a feed pump, and a temperature control unit downstream of the feed pump are fluidly connected to one another via a common pressurized water circuit. Process control using the pressurized water circuit, in which the process water is subjected to a pressure of over 2 bar and a temperature of over 100 °C, reduces vacuole formation in the granules.

[0006] In order to safely open the granulation unit for inspection or cleaning purposes or for a batch changeover, it is necessary to first drain any remaining, pressurized process water from the granulation unit. For further granulation, the missing process water must be reinserted into the system and then the entire pressurized water circuit must be reconditioned according to the specified pressure and temperature values. Due to the associated increased water consumption and the high energy requirements for the renewed pressure and temperature adjustment, the overall efficiency of the process is reduced. To at least mitigate temperature-related energy losses, it is known to integrate heat exchanger units into the process chain; however, this in turn involves increased acquisition and maintenance costs and an overall more complex device design.

[0007] Description of the invention

[0008] The invention is therefore based on the object of designing a device of the type described at the outset in such a way that, even taking into account any downtimes during which a safe opening of the granulation housing is necessary for inspection or cleaning purposes or during a batch changeover, an overall energy-efficient and water-saving production of a vacuole-free granulate is made possible.

[0009] The invention solves the stated problem by providing a heating pressure vessel between the dehydration unit and the feed pump, with which the granulation unit is fluidly connected via a return line.

[0010] These measures allow the granulation unit to be opened safely without interrupting the pressure circuit. The return of the granulate-free process water initially remaining in the granulation unit via the return line to the hot-pressure vessel prevents process water losses. Furthermore, the process water returned to the hot-pressure vessel is still subjected to such pressure and temperature that only minor treatment in the hot-pressure vessel—i.e., adjusting the process water to specified pressure and temperature values—is required.Consequently, the measures according to the invention create the conditions for water-saving and, due to the lower heat losses of the process water, also energy-efficient operation of an underwater granulation device that provides a pressurized water circuit with a process water pressure of over 2 bar, preferably over 4 bar, and a process water temperature of over 100 °C. Because the heating pressure vessel performs the main work of maintaining or adjusting both the process water temperature and the process water pressure, not only can the feed pump be relieved of its load as a pressure generator, but the use of a separate temperature control unit for the process water is also eliminated. To build up the process water pressure, the heating pressure vessel can alternatively comprise a pressure build-up valve.In addition, in the devices known from the prior art, a separate temperature control unit is usually arranged downstream of a feed pump in relation to the pressurised water circuit. However, it has been shown in practice that the steam pressure arising during the temperature control of the process water can expand upstream and thus back to the feed pump, whereby the function of the feed pump is impaired due to the steam pressure acting against its feed direction and the overall efficiency of the device is thus reduced. For this reason, the invention provides that the heating pressure vessel is arranged in the pressurised water circuit between the dehydration unit and the feed pump, i.e. that the feed pump is arranged downstream of the heating pressure vessel.As a result, the steam pressure generated during the tempering of the process water expands back into the dehydration unit, although this has been shown not to be detrimental to a reliable and energy-efficient granulation process. This also eliminates the need for a check valve in the flow connection between the dehydration unit and the boiler, which would reduce process efficiency due to friction losses. For safety reasons, however, it is recommended to install a pressure relief valve on the heating pressure vessel itself to prevent unwanted pressure peaks in the heating pressure vessel.

[0011] Known dehydration units comprise a housing with a screen basket inserted therein, acting as a separating device, into which the process water containing the granulate enters, preferably tangentially to the side wall of the screen basket in a horizontal direction. Such dehydration units typically have air flow inlets and outlets to improve the separation of the process water from the granulate, so that a negative pressure profile can be set in the housing of the dehydration unit, particularly one that favors the dehydration process. However, this has a detrimental effect on an energy-efficient underwater granulation process due to the associated pressure and possibly temperature losses.Against this background, it is proposed that the dehydration unit has a pressure-tight housing, which on the one hand comprises an inlet for supplying the granulate-carrying process water and an outlet for discharging the separated process water into the heating pressure vessel, and on the other hand a pressure lock for the separated granulate. Due to the fact that the housing of the dehydration unit is pressure-tight and therefore has no separate air flow inlets or outlets, the separation of the process water can take place at a virtually constant process temperature of over 100 °C and a virtually constant process pressure of over 2 bar, preferably over 4 bar, whereby it has surprisingly been shown that the granulate only has small amounts of residual moisture. In order to remove the residual moisture from the granulate for post-treatment, e.g.In order to feed the granules to vibrating screens without causing an undesirable pressure drop in the dehydration unit, the housing has a pressure lock for removing the separated granules.

[0012] In this context, particularly favorable design conditions and improved process conditions arise when the housing interior of the dehydration unit is penetrated by a separating plate, which, on the one hand, divides the housing interior into a dehydration chamber and a pressure compensation chamber connected to it via a pressure compensation pipe, and, on the other hand, forms a guide element for the separated process water to the outlet. As a result of these measures, the housing base itself does not have to form a guide element, preferably sloping downwards from the housing interior wall. Instead, a preferably rotationally symmetrical housing into which a separating plate is inserted can be used for simple production conditions.Due to the pressure equalization pipe, an undesirable overpressure in the dehydration chamber with respect to the pressure equalization chamber can be avoided, whereby the risk of deformation or even damage to the separating floor acting as a guide element is also reduced.

[0013] The invention also relates to a method for operating a device according to the invention. To form the pressurized water circuit, the process water is subjected to a predetermined pressure and a predetermined temperature. In a granulation step, granules are added to the process water in the granulation unit, fed to the dehydration unit, and there, the granules are separated from the process water under pressure. The separated process water is treated in the hot-pressure vessel by being readjusted to the predetermined values ​​in terms of temperature and pressure as required. The treated process water is then fed to the granulation unit. In order to be able to process the process water for inspection or cleaning purposes, or for maintenance work,In order to enable safe opening of the granulation unit during a batch changeover, the process water is passed through the bypass line bypassing the granulation unit in an opening step, after which remaining granulate-free process water from the granulation unit is passed into the heating pressure vessel via the return line.

[0014] Brief description of the invention

[0015] The subject matter of the invention is illustrated by way of example in the drawing. Fig. 1 shows a schematic process diagram of a device according to the invention and

[0016] Fig. 2 is a sectional view of a dehydration unit of the device according to the invention.

[0017] Ways to implement the invention

[0018] A device according to the invention for pressurized underwater granulation of plastics at a process water temperature of over 100°C comprises a granulation unit 1, a dehydration unit 2 for separating the process water from the plastic granules, and a feed pump 3, which are fluidly connected to one another via a common pressurized water circuit 4. A bypass line 5 is provided in the pressurized water circuit 4 for the granulation unit 1, and a heating pressure vessel 6 is provided between the dehydration unit 2 and the feed pump 3. The granulation unit 1 is fluidly connected to the heating pressure vessel 6 via a return line 7.

[0019] To form the pressurized water circuit 4, the process water is subjected to a specified pressure of over 2 bar, preferably over 4 bar, and a specified temperature of over 100 °C, depending on the plastic to be processed. For example, to produce polycarbonate granules that are as vacuole-free as possible, it is recommended to set the process water temperature in the range of 100° - 150 °C, preferably 110 - 140 °C, in particular 120 - 130 °C, which is thus within the usual glass transition range or slightly below the usual glass transition range of polycarbonates.

[0020] To achieve the desired flow patterns between the units, switching or branch valves 8, 9, and 10 can be used, for example. Thus, the bypass line 5 can be connected to the feed pump 3 via the switching valve 8, on the one hand, and to the dehydration unit 2 via the switching valve 9, on the other hand, by switching the switching valve 8 directly to the switching valve 9. However, to connect the granulation unit 1 to the pressurized water circuit 4, the switching valve 8 must first be switched to the switching valve 9 via the switching valve 10.

[0021] Before any granulation step, the pressurized water circuit 4 initially runs via the bypass line 5, the dehydration unit 2, the heating pressure vessel 6, and the feed pump 3 through the appropriate position of the switching valves 8, 9, whereas the granulation unit 1 is excluded from the pressurized water circuit 4. Melt is fed to the granulation unit 1 via an extrusion unit (not shown in detail), so that in the granulation step, with the granulation unit 1 closed and the appropriate switching position of the switching valves 8, 9, 10, the melt is cut into granules and added to the process water in the pressurized water circuit 4.

[0022] The granules are then separated from the process water under pressure in the dehydration unit 2. The separated process water is then passed to the hot-pressure vessel 6, while the separated granules are discharged via a pressure lock 11 and can be subjected to post-treatment, for example, on vibrating screens (not shown), to remove residual moisture.

[0023] The separated process water is passed to the heating pressure vessel 6, where it is treated, if necessary, to compensate for minor pressure and heat losses by adjusting the process water in the heating pressure vessel 6 to the specified temperature or pressure. For this purpose, the heating pressure vessel 6 can be equipped with heating elements and a pressure build-up valve 12. Furthermore, the heating pressure vessel 6 has a pressure relief valve 13 to prevent undesirable pressure build-up.

[0024] From the heating pressure vessel 6, the process water is transported via the feed pump 3 to the granulation unit 1, where newly cut granules are added to the process water. In order to enable safe opening of the granulation unit 1 for inspection or cleaning purposes or during a batch changeover, the process water is conducted in an opening step through the bypass line 5 bypassing the granulation unit 1, with the switching valve 8 being switched directly through to the switching valve 9. Remaining granulate-free process water can then be fed from the granulation unit 1 into the heating pressure vessel 6 via the return line 7 when the switching valve 10 is in the appropriate switching position. A pressure build-up valve 14 can be used to support this. In order to be able to reliably initiate a new granulation step, a corresponding pressure relief valve 15 can be provided to avoid pressure peaks.

[0025] Fig. 2 shows a schematic section through a dehydration unit 2 according to the invention. The dehydration unit 2 has a pressure-tight housing 16 in which a screen basket 17 is located. An inlet 18 for supplying the granulate-carrying process water opens into the screen basket 17 tangentially to the side wall of the screen basket 17. On the underside of the housing 16 there is an outlet 19 for discharging the separated process water into the heating pressure vessel 6, as well as a pressure lock 11 for the separated granulate. The housing interior is penetrated by a dividing plate 20, which divides the housing interior into a dehydration chamber 21 and a pressure equalization chamber 23 fluidly connected to the dehydration chamber via a pressure equalization pipe 22. Furthermore, the dividing plate 20 forms a guide element for the separated process water to the outlet 19.

Claims

Patent claims 1. Device for the pressurized water underwater granulation of plastics at a process water temperature of over 100 °C, wherein a granulation unit (1), a dehydration unit (2) for separating the process water from the plastic granulate and a feed pump (3) are fluidly connected to one another via a common pressurized water circuit (4), and wherein a bypass line (5) is provided in the pressurized water circuit (4) for the granulation unit (1), characterized in that a heating pressure vessel (6) is provided between the dehydration unit (2) and the feed pump (3), to which the granulation unit (1) is fluidly connected via a return line (7).

2. Device according to claim 1, characterized in that the dehydration unit (2) has a pressure-tight housing (16) which, on the one hand, comprises an inlet (18) for supplying the granulate-carrying process water and an outlet (19) for discharging the separated process water into the heating pressure vessel (6), and, on the other hand, a pressure lock (11) for the separated granulate.

3. Device according to claim 2, characterized in that the housing interior of the dehydration unit (2) is penetrated by a dividing floor (20) which, on the one hand, divides the housing interior into a dehydration chamber (21) and a pressure compensation chamber (23) fluidly connected to the latter via a pressure compensation pipe (22), and, on the other hand, forms a guide element for the separated process water to the outlet (19).

4. Method for operating a device according to one of the preceding claims, characterized in that granules are added to the process water in a granulation step in the granulation unit (1), fed to the dehydration unit (2) and there the granules are separated from the process water under pressure, which is processed in the heating pressure vessel (6) and in turn fed to the granulation unit (1), and that in a Opening step the process water bypasses the granulating unit (1) bypass line (5), after which remaining granulate-free Process water from the granulation unit (1) is fed into the heating pressure vessel (6) via the return line (7).

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