Texplast - new granulating technology makes PET recycling more efficient

by Dipl.-Ing. Gunter Schippers, NUGA AG Kunststoffschneidmuehlen, Balgach/Switzerland

Texplast GmbH (Wolfen, Germany), one of Europe's leading PET recycling firms, processes used PET one-way and multi-trip bottles, preforms and the waste material from bottle manufacturing operations. The ground material, in regranulated or agglomerated form, is re-used to make, e.g. PET sheets, preforms (bottle-to-bottle), fibres, geotextiles and packaging tapes.

PET processing - granulation a key to success

The PET bottles are supplied loose or in the form of compacted bales or briquettes (Fig. 1) from almost every part of Europe. The proverbial bottleneck for economical processing lies in reducing the material to flakes. At this stage, considerable sums of money can be lost. In addition to throughput, the predominantly decisive criteria for low-cost processing lie in the specific amount of energy consumed, the endurance or replacement cycles of the cutter blades and the time taken to replace them.

At Texplast, the granulation of PET bottles has so far been carried out using a conventional granulator with a rotor of 800 mm in diameter and 1200 mm in depth and a drive motor producing 132 kW / 180 hp. With this machine, the throughput achieved is between 1200 kg per hour (one-way bottles) and 1500 kg per hour (multi-trip bottles).

Within the framework of a project to achieve a marked increase in capacity and an improvement in productivity, the company now proposes to use a new granulating technology and, in this way, logically to optimise all its processing lines with the objectives of reducing energy consumption while increasing throughput and, at the same time, achieving savings in the costs entailed in wear and maintenance. The principal element of the project consists of two new CentriCut 44s-series granulators (Fig. 2) made by NUGA AG of Balgach (Switzerland).

CentriCut - high throughput, low energy consumption and high quality granules

The PET bottles are directed into the CentriCut machines, by way of a conveyor belt, as whole bottles in bulk, released from bales or from briquettes. By means of a horizontally arranged screw conveyor, the material is forced axially, and in a controlled way, onto the rotor.

The centrifugal force generated by the spinning rotor throws the material through the open rotor onto the granulating chamber wall (consisting of a screen and the stator blades) which surrounds the fully enclosed chamber (Fig. 3). Here, it is ground between the rotor and stator blades into cleanly cut, almost dust-free granulate which, thanks to its outstanding 'flowability' is immediately suitable for further processing.

The flakes, to which the ground bottles have been reduced, are now rapidly extracted through the large screen (electropolished and thus wear-resistant) surrounding the granulating chamber by means of a powerful, integrated vacuum extraction system. The exceptionally short dwell time of the granulate in the machine results in low levels of specific energy consumption and blade wear, and extremely high throughput rates.

The technique of feeding the material onto the rotor from inside eliminates the usual tendency of lightweight PET bottles to 'dance' on the rotor or the ejection of partially granulated material (so-called 'pumping') which occurs with conventional granulators. The rotor is of an open design, with the result that no heavy masses need be moved. This offers a further benefit in the form of reduced energy consumption.

An electronic load-dependent control system between the rotor and the screw conveyor shaft prevents the granulating chamber from becoming overfilled and material from building up against the screen, as well as permitting startup from a standstill with residues of material still in place. By contrast, it is usually necessary for the granulating chamber of conventional machines to be emptied before they are restarted.

Depending on the condition of the returned PET bottles, the size of the rotor and the pattern of screen perforations, the throughput of the CentriCut 44s is around 1000 kg per hour. The drive power required to achieve this is only 55 kW / 75 hp.

The innovative material feed system and rotor design ensure that the granulator is of a compact, energy saving configuration. The rotor is only 440 mm in diameter and 272 mm in depth.

Through the use of two CentriCut 44s running in parallel, Texplast achieves more adaptability than before, together with a considerable improvement in operating reliability and simultaneous cost savings.

High quality cutter blades and ultra-fast blade replacement substantially improve productivity

Due to its high strength, PET makes exacting demands on the cutters employed. Accordingly, it is necessary to achieve the optimum in terms of hardness and durability so that the blades are both hard enough to last for prolonged periods of time and durable enough to survive the possible effects of foreign bodies without breaking off.

The rotor and stator blades consist of identical, precisely fitting, adjustable hard metal cutters made of special steel, mounted to special supports. Three or five rows of blades are mounted around the circumference of the CentriCut rotor, while four rows of stator blades are arranged in the granulating chamber wall. To permit the rapid replacement of blades, the complete supporting assembly, including the cutting blades, is replaced. Depending on the size of the machine, this permits a complete knife replacement in only 15-30 minutes which, by comparison with conventional granulators, affords a considerable time saving. No adjustment of the gap between cutters is required, since the amount of clearance is constant over the entire length of the blades, thereby precluding adjustment errors. Blades damaged by foreign bodies can be individually replaced in the minimum of time.

The integral extraction system permits the fastest possible removal of the granulate and the rapid cleaning of the machine. This is particularly advantageous when the colour or type of material is changed. The granulating chamber can be opened without the use of tools by means of sideways swivelling screen baskets, which also make it possible to replace the screens in the minimum of time to accommodate, e.g. a change in the desired grain size. The machines are fitted with a fully integrated, safety type, noise insulated housing.

According to Torsten Kreiseler, Head of Production at Texplast, "The granulating of PET bottles, in particular, is an especially exacting operation. First and foremost, the physical characteristics of PET impose heavy demands on the granulating machines employed. Even the intake of loose bottles (and their proneness to 'dancing' in the case of conventional granulators) have a lasting effect on processing costs. In conjunction with NUGA AG, we have specially adapted the CentriCut 44-series of granulators to process PET bottles in the optimum way. In this case, the decisive arguments for our buying decision were the underlying design principle (which is distinctly different from conventional granulators) and its associated advantages.“

Conventional CentriCut 44s

granulator granulator

Throughput [kg/h] 1.200 - 1.500 1.000 - 1.200

Drive power [kW / hp] 132 / 180 55 / 75

Screen hole diameter [mm] 8-10 8-10

Blade replacement time [mins] 360 15-30

Tab. 1:

Granulation of PET bottles - comparison between a CentriCut and a conventional granulator

NUGA AG has been making granulators and accessories for granulating plastics for customers throughout the world for over fifteen years. They are primarily used as central granulators for re-processing purposes in two-stage shredding plants and for special applications in shredding bottles (PP, PET etc.). The company employs about thirty people.

For more information, please visit:

href="https://edir.bulk-online.com/oldedirredirect/203003.htm" target="blank">https://edir.bulk-online.com/oldedirredirect/203003.htm

See Figures below:

Fig.1: PET bottles – in briquette form

Fig. 2: CentriCut 44 with conveyor belt, integrated noise insulation and switchgear cabinet

Fig. 2a: CentriCut 44 - exploded drawing: 1. Screw conveyor, 2. Granulating chamber, 3. Rotor, 4. Rotor blade, 5. Stator blade, 6. Extraction system, 7. Screens (Factory illustrations: NUGA AG, Balgach, Switzerland)

Fig.1: PET bottles – in briquette form

Attachments

nuga_pet-bottles_news (GIF)

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