Apparatus and process for making crosslinkable polyethylene compounds

Abstract

The invention relates to a device and a method for producing crosslinkable polyethylene compounds. In particular to a device for producing a crosslinkable polyethylene compound, comprising a melting machine (101), a melt pump (102) and a filter unit (103). The apparatus allows the production of cross-linkable polyethylene compounds, which can be further used in the manufacture of insulating parts for medium, high and ultra-high voltage power cables. Further provided are methods for making crosslinkable polyethylene compounds.

Description

[0001] This application is a divisional application of Chinese Patent Application No. 201780006567.1, filed on January 11, 2017, entitled "Apparatus and method for manufacturing cross-linkable polyethylene compounds". TECHNICAL FIELD

[0002] The present invention relates to the technical field of apparatuses for manufacturing polymeric compounds. In particular, the present invention relates to an apparatus for manufacturing cross-linkable polyethylene compounds and to a method for manufacturing cross-linkable polyethylene compounds. BACKGROUND

[0003] Cross-linkable polyethylenes (XLPE) are used for manufacturing the insulating parts of power cables, in particular medium, high and extra-high voltage power cables. Since power cables are usually buried underground for several decades, the insulating parts must exhibit constant chemical and mechanical properties over long periods of time. One of these required properties, thermal ageing stability, is related to the persistence over time of the best insulating and electrical properties and is of fundamental importance.

[0004] Document US2009110833 provides an example of an apparatus for manufacturing cross-linkable polymeric compounds. EP2918388 provides another example of an apparatus for recycling plastic materials.

[0005] In order to produce cross-linkable polyethylenes that can allow to provide such properties, in particular to produce high quality cross-linkable polyethylenes that comply with technical and legal standards allowing their use for manufacturing the insulating parts of medium, high and extra-high voltage power cables, it is necessary to carry out a process of mixing raw materials, such as low density polyethylene (LDPE), additives and peroxides.

[0006] Usually, this process is carried out by using so-called conventional medium-high- extra-high voltage cross-linkable polyethylene compounds production lines, which are a mature technology starting in the 60s. Such production lines always require quite large facilities to ensure the production environment required by the process, which must be kept at a high level of cleanliness. This high level of cleanliness means that so-called "clean rooms" are used, in which technologies are provided to avoid the presence of contaminants and several technologies are provided to continuously monitor and maintain the high level of cleanliness required.

[0007] As far as the process carried out is concerned, the production lines or apparatuses of the prior art, when designed to produce cross-linkable polyethylenes that comply with technical and legal standards allowing their use for manufacturing the insulating parts of medium, high or extra-high voltage power cables, implement the principle that, since the peroxides that must be introduced at a certain stage of the process cannot meet the mixing temperature required, they must be subsequently introduced during a so-called soaking process.

[0008] This principle means that traditional production plants, in particular those designed to produce cross-linkable polyethylene that complies with the technical and legal standards allowing it to be used to make insulating parts of medium, high or extra-high voltage power cables, are always based on an intermittent peroxide immersion process with controlled heating, cooling and dwell time sequences. This immersion process is always carried out in a closed immersion tower, the height of which is usually about 50 m, since the chain of operations carried out inside the tower relies on gravity and such a height is necessary.

[0009] The existing technology plants for manufacturing high quality cross-linkable polyethylene therefore present a first drawback, since depending on the specific method they must always have at least one very high immersion tower. However, obtaining the building permit to build such a high tower is not always easy depending on the region of the world. Moreover, there are certainly regions in the world where the construction of relatively high towers is strictly prohibited.

[0010] In terms of costs, the existing technology plants for manufacturing high quality cross-linkable compounds present a second drawback, since they are quite expensive, with prices in the range of 15 to 20 million US dollars, including land, complete mixing plant, immersion unit and related buildings. These numbers are also intrinsic to countries where international units are established. Obviously, such a huge capital investment is largely influenced by the cost of building the immersion tower.

[0011] In addition to the need for an immersion tower and the related costs, traditional production lines also present other drawbacks, such as non-uniform dispersion of peroxide, high risk of external contamination, need to have many clean rooms and related specialized personnel, difficulty in maintaining high levels of cleanliness and further drawbacks in terms of energy consumption, which obviously leads to a relatively large carbon footprint. SUMMARY

[0012] The present invention provides a new plant for manufacturing cross-linkable polyethylene compounds, which does not have any of the drawbacks known in the art.

[0013] In particular, the present invention aims to provide a plant for manufacturing cross-linkable polyethylene, in particular of the quality that complies with the legal and technical requirements allowing it to be used to make insulating parts of medium, high or extra-high voltage power cables, which does not require any immersion tower.

[0014] The present invention also provides to build a less expensive plant for manufacturing high quality cross-linkable polyethylene.

[0015] These objectives are achieved by the plant defined in claim 1 and by the method defined in claim 7.

[0016] By using specific components arranged in a specific order, the device according to the present application, in particular when it has to be used for cross-linkable polyethylenes which must be used for the insulating parts of medium, high or extra-high voltage power cables, does not require any immersion tower. In fact, the presence of specific components arranged in a specific order allows to reduce the temperature of the polymeric compound from a very fine filtration level which must be set on the melting machine to allow the production of cross-linkable polyethylenes which can be used for the manufacture of insulating parts of medium, high and extra-high voltage power cables, to a lower level which allows the introduction of epoxide into the compound without triggering degradation reactions.

[0017] Further advantages are provided by the detailed description of the specific embodiments defined by the dependent claims. BRIEF DESCRIPTION OF DRAWINGS

[0018] The objects and advantages of the present application will be realized and attained by the apparatus particularly pointed out in the written description and claims hereinafter and the appended drawings.

[0019] - Figure 1 Figure shows a device according to a first embodiment of the present application;

[0020] - Figure 2 Figure shows a device according to a second embodiment of the present application;

[0021] - Figure 3 Figure shows a device according to a third embodiment of the present application;

[0022] - Figure 4 Figure shows a device according to a fourth embodiment of the present application;

[0023] - Figure 5 Figure shows a device according to a fifth embodiment of the present application. DETAILED DESCRIPTION

[0024] As can be seen from the figures, the device 100 for manufacturing cross-linkable polyethylene compounds according to the present application defines a production path starting from the point of entry of the device where the raw materials are acquired. Depending on the final product to be output by the device, the raw materials can be low density polyethylene (LDPE), very low density polyethylene (VLDPE), ethylene propylene rubber (EPR), ethylene-vinyl acetate (EVA), ethylene propylene diene monomer (EPDM M grade) rubber or other copolymers (EVA, EEA, EMA, EBA) or any of the above polymers and copolymers pre-stabilized with one or more antioxidants or combinations thereof. Each of the above polymers and copolymers can be pure. They can be used individually or in combination with other polymers or copolymers. The raw materials can be produced at the same location of the device, conveyed into the device through pipes connected to the reactor area, through one or more intermediate buffers or silos, or they can be delivered through a dosing table of the usual 25 kg bags. Generally, the raw materials are preferably delivered to the device in the form of granules or even in the form of a powder.

[0025] Along the production path, the device 100 according to the present application comprises several components arranged in a specific order. All the components along the production path are connected together through connection means such as pipes, ducts or any other equivalent connection means known in the art of polymeric compound production facilities.

[0026] According to a first embodiment of the present application, as shown in Figure 1 the device comprises a melting machine 101 which acquires, melts and mixes the raw materials. The melting machine 101 allows the preparation of a polymeric formulation by heating, mixing and / or blending the polymers and additives in the molten state. These polymers and additives are generally dosed automatically with fixed set points through feeders known as loss-in-weight feeders or volumetric feeders or even side feeders. The melting machine 101 is intended to output a homogeneous molten compound in which the distribution and dispersion of the additives, if present, and the shear rate and temperature of the molten compound are optimal.

[0027] Having the melting machine 101 heat the polymeric compounds at a predetermined temperature and being equipped with several inlets allows mixing different polymers or adding additives, such as antioxidants or conductive carbon black, to the polymeric compounds at any time when a specific formulation is required. For example, water tree resistant compounds or DC (direct current) compounds or semiconductive compounds can require the addition of several polymers and / or solid or liquid components. If the concentration of the additives added to the formulation is between 0 and 50%, nitrogen covering is not required.

[0028] The melting machine 101 can be an internal mixer, a twin-screw co-rotating extruder, a twin-screw counter-rotating extruder, a continuous mixer, a co-kneader or any other type of melting machine known in the art of production of polymer compounds.

[0029] Optionally, a de-bag unit (not shown) for opening and discharging the raw material bags before the melting machine 101 is arranged to feed the melting machine 101.

[0030] Once the compound passes through the melting machine 101, it is pumped by the melt pump 102 and extruded through the filtering unit 103. The melting temperature set on the melting machine 101 depends on the filtering level, but is typically about 200°C, in case the desired product provided by the plant is a cross-linkable polyethylene of mass quality that complies with legal and technical requirements allowing it to be used for the production of medium, high or extra-high voltage power cable insulation parts. This is much higher than the temperature at which peroxides degrade. As known to the person skilled in the art, such high temperatures would prohibit the introduction of peroxides at this stage of the process, and therefore, as described below, peroxides must be introduced otherwise during the process.

[0031] The input of the melt pump 102 is directly connected to the output of the melting machine 101. The melt pump builds pressure and provides a constant volume output to the filtering unit 103. The gears of the melt pump 102 are filled by the melting machine 101 by suction side and the melt pump 102 discharges a constant volume of melt to the filtering unit 103. The process is continuous.

[0032] The filtering unit 103 filters the compound coming from the melt pump 102. Preferably, the input of the filtering unit is directly connected to the output of the melt pump.

[0033] Preferably, the filtering unit 103 ensures a filtration of 35 mesh to 500 mesh, which ensures a filtration of 25 pm to 500 pm size. The filtering unit 103 can provide a greater range of filtration depending on the quality of the insulation product required in relation to medium or extra-high voltage. Thus, the filtration range can span from about 10 microns up to about 500 microns.

[0034] Preferably, the filtering unit 103 comprises a screen changer technology, such as a continuous plate, a rotating screen changer, a sliding plate screen changer or any candle filter with a woven or non-woven filter medium that can reject a size range of 10 pm to 500 pm, preferably 25 pm to 500 pm, of particles, as well as high polyethylene molecular weight called gels.

[0035] In contrast to the prior art devices, once the compound has passed through the filtering unit 103, it enters a cooler 104. The role of the cooler 104 is to reduce the temperature of the compound to a level that allows the introduction of peroxide into the compound without initiating a peroxide cleavage reaction. The cooler 104 is simply one of several melt coolers known in the art. The principle of the cooler is such that the melt flows in a single stream through a dense array of pins or teeth while a cooling medium flows inside the pins or teeth and the covering jacket. This results in a high heat exchange area allowing efficient cooling over a relatively short length.

[0036] Furthermore, the cooler 104 is connected to a mixer 106. Preferably, the output of the cooler 104 is directly connected to the input of the mixer 106. Alternatively, the output of the cooler is connected to the input of the mixer through additional connecting means such as pipes or conduits. The mixer 106 is a static mixer or a melt blender or any machine described above, comprising four to six mixing elements, the role of which is to homogenize the polymer melt in the radial direction. This ensures a high degree of mixing of the melt to result in a high quality end product.

[0037] The mixer 106 does not have any moving parts, i.e. it is a stationary or static mixer. This allows for low energy consumption, no maintenance and does not pose any risk in terms of leakage. It further allows for a predictable homogenization of the polymer compound, which is relatively inexpensive and thus provides a faster return on investment.

[0038] The device 100 further comprises an additive dispensing unit 105, which is connected to the cooler 104, the mixer 106 and / or the connecting means, such as pipes or conduits, connecting the cooler 104 and the mixer 106 together. Alternatively, when it is necessary to produce XLPE for medium voltage cables, the additive dispensing unit 105 can be directly connected to the melt machine 101 or use a feeder (not shown) described in

[0020] .

[0039] The role of the additive dispensing unit 105 is to allow the introduction of peroxide and / or antioxidant and any liquid / melted solid used for water tree retardant or DC or semiconductor formulations into the polymer compound in liquid form. The peroxide can be added as a premix of peroxide. The antioxidant is added as an antioxidant package which can be prepared beforehand. Once the melt leaving the filtering unit 103 is cooled by the cooler 104 to a peroxide compatible temperature, about 120°C, both the antioxidant and the peroxide in liquid form can be injected down into the melt by, for example, metering pumps. In terms of peroxide degradation, temperatures from about 105°C to about 140°C are acceptable and thus compatible with the process of the present application. In case a pre-stabilized polymer is used or free antioxidant is added directly into the melt machine 101, no additional antioxidant is required at this stage. In this case, no nitrogen blanket is required either.

[0040] Figure 2 An apparatus according to a second embodiment of the present application is shown.

[0041] The apparatus shares similar components with the apparatus according to the first embodiment, but it further comprises a granule sorting unit 107 arranged before the melt machine 101. Alternatively or cumulatively, an additional granule sorting unit (not shown) is arranged downstream of the mixer 106, preferably just before the final product, for example, XLPE granules are transported to a packaging unit which packages the final product.

[0042] In this embodiment, the raw material is thus first passed through the granule sorting unit 107 which has the role of purifying the raw material used by the apparatus. The granule sorting unit 107 can be set so that any granule containing contaminants larger than 60 μm is discarded before being introduced into the melt machine 101. To achieve this, the granule sorting unit 107 comprises one or more CCD cameras, one or more video cameras, X-ray, ultraviolet or infrared detection means in order to detect granules containing contaminants and, in order to discard the unwanted granules, a series of air nozzles.

[0043] According to a third embodiment of the present application, as shown in Figure 3 the apparatus shares similar components with the apparatus according to the second embodiment, but it further comprises an underwater pelletizing unit 108, a drying unit 109 and a packaging unit 110.

[0044] The underwater pelletizing unit 108 is fed by the molten compound coming from the mixer 106 and extruded through a die plate. When the compound emerges from the die plate, it is cut into granules in a cutting chamber by rotating blades and solidified underwater by flowing through a die face in the cutting chamber.

[0045] The pellets are then conveyed to a drying unit 109, for example a centrifugal drier, where the residual water is removed to output dried pellets.

[0046] Furthermore, the packaging unit 110 is adapted to package the product, i.e. the pellets leaving the drying unit 109, for example in a van box or an octabin. In order to prevent dust from entering the packaging unit, the octabin can be assembled outside the packaging unit 110. The octabin enters the packaging unit 110 through an airlock and the pellets are discharged into the octabin. Further the box is barcoded, identified, given a mix number and transferred to an automatic packaging station before being stored.

[0047] Figure 4 An apparatus according to a fourth embodiment of the application is shown.

[0048] The apparatus according to the fourth embodiment shares similar components with the apparatus according to the first embodiment, but the cooler 104 and the mixer 106 are replaced by a second melting machine 111. The second melting machine 111 is equivalent to the first melting machine 101, but it does not work at a temperature of 190°C, but at a temperature gradient from close to 190°C at its input to close to 130°C at its output. The compounds leaving the filtering unit 103 are directly fed into the second melting machine 111.

[0049] Furthermore, the apparatus comprises an additive dispensing unit 105, which is connected to the second melting machine 111, preferably close to its end.

[0050] Since the melting machine is rather long, the temperature of the compounds decreases over the length of the second melting machine 111, which allows the introduction of peroxides.

[0051] Preferably, the pellets sorting unit 10 is also arranged before the first melting machine 101. Alternatively, the apparatus according to the fourth embodiment of the application does not comprise any pellets sorting unit.

[0052] According to a fifth embodiment, as Figure 5 shown, the apparatus shares similar components with the apparatus according to the fourth embodiment, but the underwater pelletizing unit 108 is arranged immediately after the filtering unit 103. The output of the underwater pelletizing unit is further connected to the drying unit 109 and the second melting machine 111 is arranged immediately after the drying unit 109.

[0053] According to a sixth embodiment of the application, an arrangement similar to those of Figure 1 , 2 or 3 is provided, wherein both the cooler 104 and the mixer 106 are replaced by a single unit, such as a post-extrusion additive dispensing unit (PEAD) 112. Such a unit allows managing the temperature, while at the same time introducing additives through the additive dispensing unit 105 or any other means, and mixing the obtained product.

[0054] In all embodiments of the present application, the first melting machine 101, the melt pump 102, the filtering unit 103 and the granulate sorting unit 107 can be arranged in different order. Alternatively or additionally, any of the first melting machine 101, the melt pump 102 and the filtering unit 103 can be omitted. This is particularly true for the melt pump 102. Alternatively or additionally, two of the first melting machine 101, the melt pump 102 and the filtering unit 103 can be combined. In all embodiments, the first melting machine 101 can be replaced by an extruder.

[0055] The apparatus and method of the present application are suitable for manufacturing insulating materials containing or not containing semiconductor additives.

[0056] All embodiments of the present application allow the production of high quality crosslinkable polyethylene, in particular of a quality that meets legal and technical requirements allowing its use for the production of medium, high or extra high voltage cable insulating parts, without using any soaking tower.

[0057] Example 1

[0058] The crosslinkable polyethylene manufactured according to the method of the present application was evaluated for peroxide / antioxidant dispersion and compared to the peroxide dispersion of a commercially available crosslinkable polyethylene sample as follows:

[0059] According to the arrangement of Figure 1 A low density polyethylene (LDPE) was produced. The antioxidant and the peroxide were pre-mixed before injection and added in liquid form by the additive distribution unit 105. The temperature of the molten LDPE compound was 115°C to 120°C.

[0060] 50 to 100 of the obtained granules were collected, molded into a film in a 500 pm thick special frame at 115°C and rapidly cooled.

[0061] The population of granules analyzed was scanned with a Fourier transform infrared spectrophotometer (FTIR). For both 550 cm -1 and 580 cm -1 The additive net absorbance was determined at both wavelengths. The film thickness of each scan was determined by measuring the net absorbance of the polyethylene band at 2019 cm -1 The additive absorbance was determined using the following equation:

[0062] Y = net absorbance ((550 cm -1 + 580 cm -1 ) / 2) / net absorbance (2019 cm -1 )

[0063] The following parameters are considered:

[0064] n: sample size or population

[0065] X: point estimate, e.g., sample mean

[0066] Confidence level (most commonly 95%)

[0067] The chosen confidence level provides, in turn, a confidence level coefficient taken from the Student Table.

[0068] “Stdv”: sample variability or standard deviation of the point estimate.

[0069] The following expression provides a 95% confidence interval for the population mean and can be expressed as:

[0070] X ± (Stdv * Student (95%, n-1) ( / sqr(n)

[0071] The results, relating to sample n°1 produced according to the present application, sample n°2, corresponding to a first commercial compound produced with a conventional infusion system, and sample n°3, corresponding to a second commercial compound produced using a traditional infusion system, are summarized in the following table 1 :

[0072]

[0073] Table 1

[0074] Therefore, the additive is distributed very narrowly in the compound produced according to the present application compared to the traditional infusion method.

Claims

1. Apparatus (100) for manufacturing cross-linkable polyethylene compounds, said apparatus defining a production path starting from an inlet point where the raw material is obtained, said apparatus comprising a first melting machine (101), a melt pump (102) and a filtering unit (103), characterized in that in the direction of said production path starting from said inlet point, said apparatus comprises, arranged after said filtering unit (103), a cooler (104), a mixer (106) arranged after said cooler (104) and an additive dispensing unit (105) connected with said cooler (104), with said mixer (106) and / or with connection means connecting together said cooler (104) and said mixer (106), wherein said cooler is configured to cool the molten material exiting said filtering unit (103) to a temperature comprised between 105°C and 140°C.

2. The apparatus of claim 1, wherein, in the direction of said production path starting from said inlet point, said apparatus comprises, arranged after said filtering unit, a cooler (104), a mixer (106) arranged after said cooler and an additive dispensing unit (105) connected with said first melting machine, with said cooler, with said mixer and / or with connection means connecting together said cooler and said mixer, and is characterized in that it comprises at least one particle sorting unit (107) arranged before and / or after said first melting machine (101).

3. The apparatus of claim 1, wherein, it comprises a bag removal unit arranged before said first melting machine (101).

4. The apparatus of claim 1, wherein, in the direction of said production path starting from said inlet point, said apparatus comprises, arranged after said filtering unit, a cooler (104), a mixer (106) arranged after said cooler (104) and an additive dispensing unit (105) connected with said first melting machine (101), with said cooler (104), with said mixer (106) and / or with connection means connecting together said cooler (104) and said mixer (106), and is characterized in that it comprises a underwater pelletizing unit (108) arranged immediately after said mixer (106).

5. The apparatus of claim 4, wherein, it comprises a drying unit (109) arranged immediately after said mixer (106).

6. A method for manufacturing an insulation component of a medium, high or extra high voltage power cable using a cross-linkable polyethylene compound, wherein said cross-linkable polyethylene compound is manufactured using the apparatus according to claim 1.

7. The method for manufacturing an insulation component of a medium, high or extra high voltage power cable using a cross-linkable polyethylene compound according to claim 6, wherein said cross-linkable polyethylene compound is manufactured by a method comprising the steps of: • outputting a homogeneous molten raw material using a first melting machine (101), whose output is connected to the input of a melt pump (102); • using a filtering unit (103), whose input is connected to the output of said melt pump (102), said melt pump (102) discharging a constant volume of melt to said filtering unit (103); • outputting a homogeneous molten raw material using a first melting machine (101), whose output is connected to the input of a melt pump (102); • using a filtering unit (103), whose input is connected to the output of said melt pump (102), said melt pump (102) discharging a constant volume of melt to said filtering unit (103); characterized in that The method further comprises the steps of: • using a cooler (104) whose input is connected to the output of the filtering unit (103), wherein the temperature of the melt is reduced in the cooler (104) to 105 to 140 °C to allow the introduction of peroxide into the polymer compound without initiating a peroxide cleavage reaction, a mixer (106) whose input is connected to the output of the cooler (104), and an additive dispensing unit (105) whose output is connected with the cooler (104), with the mixer (106) and / or with a connection device that connects the cooler and the mixer together, wherein the peroxide is introduced into the polymer compound in liquid form by means of the additive dispensing unit (105).

8. The method of claim 7, wherein the method does not use a soaking column.

9. The method of claim 7, wherein the feedstock is low density polyethylene, very low density polyethylene, ethylene propylene rubber, ethylene-vinyl acetate, ethylene propylene diene monomer rubber or other copolymers or any of the above polymers and copolymers pre-stabilized with one or more antioxidants or combinations thereof.

10. The method of claim 7, wherein the mixer is a static mixer.

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