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The Evolution of Modern High Capacity Pellet Classifiers

February 25, 2008
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Introduction
Circular vibratory separators have been classifying plastic pellets for well over 25 years. During this time, production rates have increased many-fold which has forced designers of this equipment to search for fundamental changes in the machine configuration in order to keep up with the ever increasing production rates.

How the Machine Operates
The basic assembly consists of a motor plus a number of interchangeable frames that contain screen-cloth decks and discharge outlets. Mounted rigidly to the main screen assembly, the motor has a double-extension shaft, which is fitted at its ends with variable eccentric weights. The screen assembly is supported on the circular base by springs that allow the assembly to vibrate freely, while preventing transmission of vibration to the floor. in properly designed units, the wire screen-cloth does not flex; thus, its life is prolonged. All parts above the spring level, including the motor, are integrated into a rigid structure. The spring''s isolation from the support base makes the system self-balancing, requiring minimum power and avoiding mechanical stress.

Typical power requirements are 1/3 hp (0.2485 kw) for 2-ft (610 mm) diameter machine, 1 hp (0.7457 kw) for a 4-ft (1,220 mm) machine, and 2 hp (1.4914 kw) for a 6-ft (1,830 mm) unit. These requirements are a function of size and construction of the separator, and not of material throughput. The motor must have sufficient low-speed torque to accelerate the eccentric weights to a speed just above the resonant frequency of the system, as well as overcome the hysteresis loss in the springs, and losses due to the slight flexing of the frames.

Material to be separated is fed to the center of the top screen. Oversize particles move to the screen periphery where they are discharged, while undersize particles or liquids pass rapidly through it. Units contain up to five frames. In multiple-frame units, each lower screen is preceded by a feed tray that redirects undersize material to the center of the screen beneath. This increases efficiency by forcing each particle to travel the full distance from the center to the periphery. Multiple-deck construction also saves operating space.

Vibratory Motion
The motion of these separators is three-dimensional. The top eccentric weight of the double-extension shaft motor is in a plane close to the center of the mass of the assembly, imparting a horizontal throw to it. The bottom weight is below the center of the mass, giving the assembly a high-frequency tilt. The third dimension of motion (tangential) results from the vector combination of the horizontal and vertical components. The tangential component helps move oversize material laterally across the screen in a spiral path while undersize particles flow down through the openings.

On circular-screen separators, top and bottom weights are independently adjustable. The mass and angle between the weights can both be varied, offering a great deal of control over the three vibrating components, and thus permitting optimization of equipment for varying conditions and materials.

Varying the Flow Patterns
Flow patterns are varied by changing either the amount of the weights or their relative position. Increasing the mass of the top eccentric weight increases horizontal throw of the screen, speeding up the rate of discharge of oversize material. This is especially useful for applications that contain large amounts of oversize solids, such as dewatering.

Adding to the bottom eccentric weight will result in a larger vertical component of motion, promoting "turnover" (tumbling of the material at the screen surface). This maximizes the quantity of undersize material passing through the screen. Increased vertical motion also inhibits blinding of the screen by near-size particles (those slightly larger than the mesh openings). When processing light or fragile solids, it is sometimes desirable to reduce the bottom weight to minimize vertical motion, thereby avoiding particle breakage and attrition.

Tangential motion of the screen is controlled by the relative angular positions of the top and bottom weights. The screening pattern, therefore, is changed by altering the relative angle of these weights. Typical flow patterns generated by various angles are shown in Fig. 1.

When the weights are aligned and move in phase, the tangential component is at a minimum, and there is no tendency for the material to spiral; travel is radially from the center. When the bottom eccentric leads the top eccentric, a spiral motion is induced.

Under some conditions, it is possible to prevent discharge of oversize particles from the screen surface. Such a flow pattern is used when there is a small amount of oversize material. Adjustment of the weights can be made in a few minutes by the operator.

Capacity of circular-screen separators may be limited either by a particular unit''s design or by what happens at the screen surface.

Historical Background
Early in the history of pellet classification, pellets were formed by extruding molten plastic into spaghetti-Like strands, which were conveyed through a water cooling bath to form solid strands, which were then cut into the desired pellet length. In this process the cutting knife did not always cut true and multiple pellets were connected together in long strings. Also, fine dust was generated in the cutting process. To ensure quality product was delivered to the customer, the undersized pellets, dust, and any oversized pellets had to be removed. It was at this separation stage that the two-deck circular Kason Classifiers were applied. The advantages of the circular pellet classifiers were:

1. Low cost
2. Ease of installation
3. Simplified piping requirements
4. Stainless steel contact parts for quality control
5. Easily adjusted for varying process conditions
6. High degree of on stream reliability.

Typical screen meshes utilized in this type of equipment were 4 mesh on the top screen and 8 to 12 mesh on the lower screen. Often when the strands of stringy multiples would curve like a snake, the 4 mesh upper screen would be replaced by a smooth-surface perforated plate with the center and outer periphery of the perforated plate left blank (not punched). This design causes the string of multiples to lie flat on the deck, preventing the curled edges of the strands from jamming into the openings, thereby minimizing cleaning requirements on the top deck.

Typical capacities for these conventional two-deck classifiers are listed below:

Conventional two-deck pellet classifier capacities
Machine Diameter
Inches (mm) Recommended Maximum Capacity
lbs/h (kg/h)
18 (450) 1,000 (453)
24 (600) 2,000 (907)
30 (760) 3,000 (1,360)
40 (1,000) 4,500 (2,041)
48 (1,220) 8,000 (3,628)
60 (1,520) 12,000 (5,443)
72 (1,830) 15,000 (6,803)

The introduction of polyolefin pellets and the new processing technique of extruding and cutting pellets under water has radically increased pellet production rates leading to increasingly efficient processing lines.

The processing of underwater-cut polyolefin pellets involves the extrusion of molten resin through a die plate equipped with a rotating knife submerged in water. Thus, the strands become pellets as they exit the die plate. The water acts to cool the pellets to the point where they develop an outer skin and also to convey the pellets to a bulk dewatering device. The bulk dewatering device removes the bulk of the water from the pellets and discharges the pellets and some water into a spin dryer which uses centrifugal force to spin the pellets dry of surface moisture. The dry pellets carrying either longs (multiple pellets stuck together) or oversize (clumps of pellets) and fines which must be removed from the on-size pellets. The early 72" (1,830 mm) diameter circular pellet classifiers found a home in this application at capacities of 15,000 lbs/hr (6,800 kg/hr) but were sometimes pushed to 18,000 lbs/hr (8,100 kg/hr) in short bursts to make up for process fluctuations. On stream reliability was high and these units performed well until capacity rates began to inch up to 20,000 lbs/hr (9,070 kg/hr) and higher. At these higher rates, product could not discharge from the lower deck quickly enough and would back up on the deck causing excess weight of pellets on the deck, ultimately causing premature screen breakage and undesirable downtime.

The Peripheral Discharge Concept
Years prior to the introduction of high capacity underwater cutting extruders in the polyolefin manufacturing process, Kason became deeply involved in the solution of high capacity dedusting problems in other industries such as animal feed pellets, urea prills, etc. Studies performed by Kason indicated that dramatic increases in capacity could be attained by discharging the product around the full 360° periphery of the machine. Careful investigation showed the following two reasons for this improvement:

1. By using a 360° peripheral discharge, the "rope" or conveying section of the screen is completely eliminated. This is significant because in heavily loaded screen decks much of the mesh near the screen periphery is devoted to conveying the overs product around the deck to the discharge spout. Little, if any, dedusting takes place under the "rope."
2. As the central feed stream fans out across the circular deck, the bed depth drops dramatically, approaching single-particle depth where near size and stubborn fines have enhanced opportunity to pass through the screen aperture.
Figure 2 illustrates how these machines were built up to contain a conventional scalping deck above the 360° peripheral dedusting deck. For example, such units installed in polyolefin lines increased screening capacity approximately 50% over the rates reported above for conventional two-deck machines. Again, the limitation to this design was material conveying. As the product rate increased it eventually became greater than the conveying capacity of the vibrating spiral trough used to convey the product to the discharge spout. When this limit was reached, product began to accumulate on the dedusting deck increasing the bad depth, reducing screening effectiveness and risking screen breakage.

Again experience gained in other process applications was helpful in understanding and then solving the problem. In many high capacity scalping operations the primary problem is not the screening operation but rather discharging the scalped product at very high rates. In these applications the discharge frame is built with a highly sloped discharge ramp leading to an oversize oblong spout (see Figure 2). The highly sloped ramp acts like a steeply inclined vibrating feeder or conveyor and, because it cuts across the diameter of the discharge frame, the length of particle travel is reduced when compared to the peripheral trough by a factor of approximately three. Combining this product discharge arrangement with peripheral discharge off the dedusting deck was recognized as the design approach to take. However, this left the problem of how to handle the fines coming through the dedusting screen. This was ultimately solved by attaching a collecting cone to the underside of the periphery of the dedusting deck and bringing the bottom of this fines collection cone through an opening in the high capacity inclined product discharge ramp. This allowed fines to discharge onto a simple dome beneath the high capacity ramp where they were then conveyed by machine vibration to the fines discharge spout. Figure 3 is a cross-section through a typical high capacity pellet classifier showing the configuration discussed above. Figure 4 is a photo looking down onto the peripheral deck and through the wire mesh onto the fines collection cone.

This configuration has raised nominal pellet production rates from 6.8 MTPH for a conventional 72-in (1,830 mm) diameter classifier to 25 MTPH for a high capacity unit of equivalent diameter. High capacity units of 84-in (2130 mm) diameter achieve rates to 35 MTPH, while 100” (2540 mm) diameter units top 50 MTPH.

The current designs are more than adequate to handle most of today''s extruders but it is recognized that as extruder capacity continues to evolve and increase, so must the classifier capacity.
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