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pelletizer - an overview | sciencedirect topics

There are two main types of pelletizer that are used to produce iron ore pellets at industrial scale, the rotary drum and the disc. Besides iron ore agglomeration, these pelletizers can also be used for other materials such as copper ore, gold ore, coal, and fertilizer [12].

The rotary drum pelletizer was first used for taconite pellets in the early 1940s [14, 18]. A large drum-shaped cylinder is slightly elevated at one end, approximately 34. The iron ore and binder mixture enters the high end and finished pellets exit the low end. A roller screen is usually attached to the exit to separate pellets within the desired range from undersize and oversize, the latter two streams being recirculated (oversize after being crushed). The recirculating load tends to be approximately 150250% by weight of feed. Although a rotary drum pelletizer requires a roller screen it provides a more complete control of size. For a drum pelletizer flow sheet, see Figure 1.2.6.

Disc pelletizers are also used extensively worldwide. The advantage of the disc pelletizer is that there is no recirculation. The desired blend is fed to the pelletizer, which is a large disc inclined at 4060 to the horizontal (Figure 1.2.7). The rotation of the disc causes the formation of seeds, which grow into full-sized pellets. Factors affecting the final pellet size include the disc angle, feed rate, water addition, and rotation speed. As the diameter of the pelletizer increases, the speed should be decreased, otherwise due to the high impact pellets will start breaking. Disc pelletizers are very simple to design and have excellent performance [13].

The edge of a network typically refers to the processor located near the machine like HVAC system, traction system in a locomotive, or a pelletizer equipment, which is being monitored or actuated. The technology has been limited to accumulating and forwarding data to the cloud. It has been underutilized thus far. What if the industrial companies could turn vast amounts of data into pragmatic intelligence, available right at the edge? This technology is essential as it is rapidly emerging as a powerful force in turning industrial machines into intelligent machines.

Most HGM compounded polymers need to be pelletized for further processing, for example, for injection molding. In a standard water bath pelletizing system, the strands after cooling enter into the cutting chamber of the pelletizer where a rotating blade cuts the strands into small pellets. In a standard pelletizer, a certain amount of HGM breakage is possible depending on the HGM collapse strength (Figure3.9).

Figure3.9. Percent of hollow glass microspheres (HGM) void volume loss due to pelletizing as a function of isostatic crush strength of HGM used in homopolymer polypropylene with a melt flow index of 4g/10min at 230C.

The small amount of HGM breakage that is observed with low-strength HGM grades during standard pelletization can be minimized, if not prevented, by an underwater pelletizer. In this process, the molten polymer is cut into droplets by the fast rotating blades of the pelletizer just as it is exiting the die hole and emerging into the process water. Since the polymer is cut when the polymer is molten, the bubble breakage is prevented.

An extruder pumps melt through a straining head into the die. It passes through round holes in its die plate where a wet atmosphere exists. Upon exiting the plate, a spinning knife blade cuts the extrudate into pellets. The pellet/water slurry is pumped into a dryer where the pellets separate from the water. Water is reclaimed for repeat use.

Very popular are the wet-cut underwater pelletizer. The die face is submerged in a water housing and the pellets are water quenched followed with a drying cycle. Throughput rates are at least up to 50,000 lb/h (22,700 kg/h). Smaller units are economical to operate as low as 500 lb/h (227 kg/h).

The hot-cut pelletizer has melt going through a multi-hole die plate. A multi-blade cutter slices the plastic in a dry atmosphere and hurls the pellets away from the die at a high speed. Usually the cutter is mounted above the die so that each blade passes separately across the die face and only one blade at a time contacts the die. Pellets are then air and/or water quenched, followed with drying if water is involved. Throughput is up to at least 15,000 lb/h (6810 kg/h).

The water-ring unit has melt extruded through a die plate and cut into pellets by a concentric rotating knife assembly. Pellets are thrown into a rotating ring of water inside a large hood. After cooling in the water, they are spirally conveyed to a water-separated and then to a drying operation.

With the rotating-die unit, a rotating hollow die and stationary knife is used. The die, which looks like a hollow slice from a cylinder, has holes on its periphery; melt is fed into the die under minimal pressure and centrifugal force generated by the die rotation causes the melt to extrude through the holes. Pellets cut as each strand passes a stationary knife are flung through a cooling water spray into a drying receiver.

In general, mixed compounds can emerge from the mixing equipment as lumps, strips, and slabs. In some cases the shapes and dimension are not suitable for use in manufacturing equipment, such as injection molding machines, extruders, blow molding, and rotomolding equipment. In order to prepare the mixed compounds for the next manufacturing step, they have to be reduced in size. The size reduction methods include pelletizing, dicing, and granulation. The processes and equipment used for pelletizing and dicing are listed in Table 4.8.

Pelletizing is the most widely used process for the size reduction of many thermoplastic materials, including raw polymers and mixed compounds. The process involves extrusion through a die. The extrudate is cooled to solidification and then cut into pellets, or the molten extrudate is cut as it emerges from the extrusion die and the pellets are subsequently cooled. In the latter case, both cutting and cooling can be done in air or water, or cutting may be done in air, followed by quenching in water [45]. Cutting thermoplastics as a melt rather than as a solid generates fewer fines and less knife wear [46].

The pelletizing device, which converts the product into pellets of different shapes and sizes, is frequently attached to an extruder or a gear pump. The pellet form offers several processing advantages over other forms. It can be easily weighed, uses simple feeding systems with fewer feeders, and allows homogeneous and uniform free-flowing feed for the subsequent end uses.

The product viscosity, heat history, uniformity, required throughput, and other factors will generally determine the cutting method that can be successfully employed. Following are the most widely used pelletizing machines used for thermoplastics.

Strand pelletizers have been the most widely used machinery for pelletizing thermoplastic polymers and compounds [47] in the past, but the current trend is to use the underwater machines. The strands emerging from the die of an extruder or a gear pump are conveyed through a cooling water bath with grooved rollers that keep them separate until they solidify. Then they are air dried and fed into nip rolls to a multiknife-blade rotor operating against fixed blades. The cross-section of the pellets can be round, oval, or nearly square [48], with typical diameter of 24mm (0.080.16in) and length of 15mm (0.040.2in). There are several methods of cooling and drying the pellets; these are discussed in some detail in Ref.[49]. A schematic diagram of a strand pelletizer is in Fig.4.20.

Die-face pelletizers produce pellets by the use of knives rotating rapidly across a multiple-hole die plate. The pellets are quenched in air alone with a water mist or by subsequent immersion into a water bath [50]. A schematic diagram of a hot die-face pelletizer is in Fig.4.21.

Underwater pelletizers are probably the most versatile equipment for a wide viscosity range, higher outputs, and pellet size capability. They operate with the die face and rotating cutter fully immersed in water. The produced pellets are carried away as slurry in water for further cooling in transit, to the dewatering, screening, and drying equipment [51]. Anexample of an underwater pelletizing head is in Fig.4 22(A), and the detail of cutting knives is shown in Fig.4.22(B).

Figure4.22. (A) Schematic drawing of an underwater pelletizing head. (a) Water chamber; (b) Pellets; (c) Knife holder; (d) Die plate; (e) Extruder feed screw. (B) Detail of the cutting head of an underwater pelletizer.

In the water ring pelletizer the molten strands emerging from the die are cut with a centrally mounted cutter. The freshly cut pellets are thrown into spirally rotated water, about 20mm (0.8in) deep. The pellets are cooled down in the water, then screened and dried. A schematic of this type of pelletizer is in Fig.4.23(A), and an example of the arrangement of die knives in such a device is in Fig.4.23(B).

Figure4.23. (A) Schematic drawing of a water-ring pelletizer. (a) Water ring chamber; (b) Pellets; (c) Knife holder; (d) Die plate; (e) Extruder feed screw; (f) Motor. (B) Detail of the cutting head of a water ring pelletizer. (C) Typical layout of a system for pelletization of thermoplastic elastomers.

In centrifugal pelletizers the melt is fed into a rotating die chamber at atmospheric pressure. The pressure to extrude the polymer through the die holes is generated by the centrifugal force within the die rather than by an extruder or gear pump. The emerging strands are cut by a single stationary knife [51]. This type of machine was offered at one time but is no longer being made.

Thermoplastic elastomers can be processed essentially by any of these machines; the selection depends mainly on the apparent viscosity of the melt of the given material. The layout of a typical pelletizing system for most thermoplastic elastomers is in Fig.4.23(C).

Dicers are used to produce square, rectangular, parallelogram, or octahedral pellets [53]. The strip from an extruder or a sheet from a calender or a roll mill is quenched before entering the dicer and is then fed at a constant rate through nip rolls into the rotating knives operating against a stationary-bed knife. The configuration of the knives, the strip feed rate, rotor speed, and the number of rotating knives determine the pellet size, configuration, and output.

Although granulation is mostly used for reclaiming and recycling of thermoplastics, granulators can be useful for short runs or for compounds that are difficult to pelletize. They have two or more rotary knives inside a chamber, where stationary knives are mounted. Granulators are used in very limited applications to pelletize hot melt directly from the extrusion die; cooling for solidification is achieved in the granulator [54]. A schematic of a granulator is in Fig.4.24.

The compounding process is defined as melt mixing different components in either a single or twin screw extruder to form a new material. This can be as simple as mixing a colorant in the form of a liquid, powder, or concentrate into the polymer melt to change the resin color, or as complicated as blending or alloying two or more different resin systems while adding a filler, reinforcement, colorant, flame retardant, and/or stabilizers to produce a radically different formulation with its own unique properties and end-use performance. Figure 44.1 shows a compounding process with feed system, extruder, strand die, water bath, air knife, and pelletizer. Molten polymer exits the die as a strand approximately 1/8 in. (3.2 mm) in diameter. Strands pass through a water bath to remove the heat and solidify the strand, which will be cut to length in the pelletizer. Between the pelletizer and the water bath is an air knife or air stripper to remove any surface moisture attached to the strand. A rougher strand surface makes it more difficult to remove the moisture after the strands exit the water bath. How many strands exit the extruder depends on the extruder size and throughput rate, with larger diameter extruders producing more strands and higher throughput rates.

Figure 44.2 shows a water bath and extruder die with a fume hood surrounding the die to remove volatiles coming off the strands before they enter the water bath. Cold water enters the water bath at the tank bottom farthest from the extruder and exits from the tank top next to the extruder. This allows the warmest water to be removed from the tank. It is common to have the water bath water flow countercurrent to the strands. This results in the water temperature being hotter at the strand entrance than at the strand exit. If insufficient cold water is supplied to remove the heat, the water bath temperature can build up over the run, necessitating a higher water flow to keep the strands cold enough so that they can be pelletized.

Strand throughput is measured in pounds per hour per strand. Typical strand throughput rates range from 25 to 125 lb/h for a 1/8-in. (3.2-mm) diameter strand and from 12 to 70 lb/h for a 3/32-in. (2.5-mm) diameter strand. A given water flow rate is required to remove the excess heat and cool the strands sufficiently for pelletizing. As the throughput rate per die hole increases, strand or water baths have to become longer or a serpentine path is required to ensure sufficient time for cooling the strand. A lower throughput per strand rate requires more strands run at slower speeds to produce the same extruder throughput rate. The time a strand spends in the water bath is easily calculated based on the polymer density and throughput rate. Assume that a polycarbonate strand with a density of 1.21 g/cm3 is running at 100 lb/h per hole, and the strand diameter is 1/8 in. How long does the strand spend in a 50-ft-long water bath? The following calculations show how to determine the time in the bath. Eqn (44.1) gives the pounds per minute per hole throughput rate:

At 1.66 lb/min per hole, there are (1.66 lb/min 454 g/lb) = equals 753 g/min per hole. With each foot of strand weighing 2.91 g, each strand in the process is running at the rate given by Eqn (44.4):

The cooling time required to adequately cool the strand depends on the strand diameter, the water temperature, and the temperature difference between the polymer and the water. The heat required to be removed (Q) is given by Eqn (44.6):

(If the result is calculated in BTUs, to convert this to other units, 1 BTU = 252 cal = 1055 J = 0.293 Wh.) In the previous example, the amount of heat (Q) that has to be removed per hour per strand is calculated in Eqn (44.7). The heat capacity of polycarbonate is 0.3 BTU/lb F.

This is then multiplied by the number of strands to determine the total heat to be removed. The heat removal efficiency depends on the bath water temperature. Heat transfer occurs more rapidly with lower water temperatures due to the large temperature difference between the material and the water. The water must be recirculated using a chiller, cooling tower, or once-through water to maintain the water bath temperature at the correct setting. If the water flow through the bath is insufficient, the water bath can heat up over time until the water actually boils. Water bath temperature can be critical to the processability of the strands. Water baths that are too hot may not provide adequate cooling when the strands enter the water to form a thick-enough skin layer on the strand. This leads to periodic strand breakage. Cold water chills the strand skin rapidly. This forms a thick skin while the center is still molten. Strands where the skin layer becomes thick very rapidly may have a vacuum void down the center. This gives pellets with holes in the center. As the center cools, the outside skin cannot be drawn in toward the center as the center material is shrinking. Continued cooling and shrinkage of the center leads to a vacuum void in the center of the strand due to the resin shrinking away from itself, filling the constant volume formed by the outside skin. Holes in the center of pellets caused by vacuum voids is not a product detriment. Optimum cooling bath temperature accompanied with the proper strand diameter can eliminate vacuum voids.

The second factor influencing vacuum voids is the polymer thermal conductivity. Thermal conductivity measures heat transfer by conduction. Higher thermal conductivity allows more rapid heat transfer from the strand center to the surface, where it is transferred to the cold water. Higher conductivity materials are less likely to have vacuum voids as the entire polymer cross-section is cooling relatively simultaneously and shrinking at the same time, without the formation of a thick solid skin. In practice, polymers are great insulators with a poor thermal conductivity, leading to vacuum void formation. As with other plastic parts, the larger the strand diameter, the more likely the strand will have a vacuum void in its center.

All the factors mentioned are important in determining the water bath size required to properly cool the product. Depending on the floor space available, a wider shorter bath can accommodate the same total extruder throughput, running more strands at a lower throughput per hole rate. Fewer strands with a higher throughput per hole may be easier to string up from the extruder die to the pelletizer.

After the water bath, an air knife or air wipe removes moisture on the strands. An air knife blows air across the strands as the strands are bent over a bar. The air knife needs to be positioned so it is not blowing moist air toward the die face and cooling the die. Depending on the water bath length, this may or may not be a problem. If the air is blowing toward the die, an air deflector can be installed over the water bath or the air knife can be turned at a slight angle so that the air blows to either side of the die. (Preferably, the air will blow to the nonoperator side.) An air stripper operates using vacuum. The break angle and strands are pulled through the air stripper in a fashion similar to that of an air knife, but air is pulled across the strands, pulling the excess water into the equipment instead of blowing it away. Water is collected and run into a drain. The advantage of an air stripper versus an air knife is that moist air is not blown into the room or toward the die. When a large amount of moisture is removed, an air knife can cause a wet spot on the floor, which is a safety hazard. Figure 44.3 shows an air knife. High-velocity air impinges the strands, blowing the water off. Figure 44.3 shows the top separated from the bottom; however, one side is connected to the bottom, and the other side is open to string up the strands through the equipment. An air stripper is very similar in appearance to an air knife except that the water is pulled off the strands into the equipment via vacuum.

After exiting the air knife or air stripper, the dry strands enter the pelletizer, where they are chopped to length. Two feed rolls, designated as an upper and a lower feed roll, are used to pull the strands into the cutting section at a constant speed. The lower feed roll is carbide or elastomer coated (depends on application) and driven by gears, belt, or chain. The upper feed roll is covered with different durometer elastomers. The material processed and its temperature determines the roll covering. A gap is set between the upper and lower feed rolls based on the strand diameter. In larger pelletizers, the upper feed roll is gear driven in synchronization with the bottom roll. In smaller pelletizers, the top roll may be an idler roll, forcing the strands against the bottom, driven feed roll, with the moving strands driving the upper roll. Either a mechanical device (in smaller pelletizers) or an air-actuated cylinder raises and lowers the upper feed roll. In operation, air is normally used to hold the top roll down at the specified gap opening, with the strands being pulled by the rollers at a constant speed as they pass between the rollers. Upper-roll wear can cause strands to slip in the pull roll, leading to dropped strands and processing problems. Roll wear is observed as grooves in the rolls or nonuniform wear across the roll where the edges may have a larger diameter than in the center.

The pull roll speed is critical to the pelletizer operation; it has to be properly synchronized with the throughput rate or the process will continuously produce unacceptable strand quality or dropped strands. High pelletizer speed draws the strands away from the extruder too fast, generating significant postdie drawing. This results in a smaller strand diameter than is desired. Excessively high pull roll speed can cause the molten polymer to break as it exits the die. Low puller roll speed causes the strands to be of a too large diameter. This results in significant quantities of rejected product because the pellets are too large. Excessively, slow speed can cause adjacent strands exiting the die to touch each other and become a double strand. If the strands touch each other coming out of the die, quite frequently, a large mass will be produced that cannot go through the pelletizer or air knife, breaking down the entire process.

Figure 44.4 shows an open view of a pelletizer with upper and lower feed rolls, bed knife, and rotor. The cutting action is a scissor cut between the bed knife and rotor. The bed knife and feed table are slightly below the feed roll centerline, pulling the strands into the pelletizer; this ensures a clean pellet cut. Rotors either have individual bolt-on blades or come as one-piece helical rotors with the number of cutting blades dependent on the rotor size and anticipated throughput or cutter speed. Small or laboratory size 4-in. diameter rotors have four or eight blades. Six-in. diameter rotors use between 14 and 36 blades, whereas 12-in. diameter rotors have between 48 and 72 blades. Helical cutter blades do not cut all the strands simultaneously as the blade progressively cuts strands across the rotor face. This reduces the energy required to cut the pellets while providing a cleaner cut with fewer fines and longer blade life. Cutting blades are normally made of high speed steel, D-2 tool steel, or carbide and stellite coating to extend blade life. Figure 44.5 shows an opened pelletizer where one can see the upper feed roll and the bolt-on blades on the rotor.

The bed knife can be rotated as it wears, allowing all four cutting edges to be used before it has to be resharpened or replaced. Rotor blades, whether replaceable or part of a helical cutter, are sharpened on the rotor to ensure that the length each blade extends from the center of the rotor is exactly the same. Bed knife rotor clearance is set between 0.001 and 0.005 in.; if the rotor blades are not all sharpened simultaneously and properly set, the cutter blade and the bed knife clearance can vary from one cutter blade to another, resulting in poor cut quality or incomplete cutting. If the clearance is too high, strands may not be properly cut, resulting in sections of plastic ribbon being generated that can clog and jam the cutter discharge chute.

To change the pellet length, some pelletizers have independent feed roll speed and rotor speed controls. As the feed roll speed relative to the rotor speed is increased, the pellet length increases. Pellet length is decreased by reducing the feed roll speed relative to the rotor speed. In practice, the feed roll speed is dictated by the extruder throughput and the strand size. After the feed roll speed is properly set to provide a stable, consistent process, the rotor speed is increased or decreased relative to the puller roll speed to change the pellet length.

After the pellets exit the pelletizer, they pass through a classifier to remove fines or large pellets created in the pelletizing process. Large pellets are minimal if the pelletizer is run at the correct speed, the cutter blades are in good condition, and the gap between the blades and bed knife is properly set. Screens are available to segregate pellets according to the product specification. Generally, two different screen meshes are used, with the top screen mesh allowing all the pellets meeting the upper size limit specification to pass through, where they drop on to a second screen. The second screen mesh size allows all pellets below the lower limit to pass through. In this manner, oversized pellets are collected off the top screen, first-quality product is collected from between the screens, and fines and undersized pellets are collected from the bottom of the classifier. All off-spec pellets can be recycled back into the product at a lower percentage or sold as second-grade product.

There are basically two classifiers that work on the principles of either vibration or oscillation to move the pellets across the screen to the exit port for collection. One classifier is rectangular and sits at a slight angle. Pellets are dropped on the highest point and vibrated down the incline. Screens make up the incline plane, allowing pellets to pass through the screens to separate the product. The second classifier is circular. The pellets are dropped onto the middle of the screen. As pellets drop through the screens, they fall onto a funnel-shaped section that delivers the pellets to the center of the next screen. Vibration moves the pellets from the center to the edge, where they are discharged into a collection chamber. Figure 44.6 shows cross-sections of both classifiers.

Once pellets exit the classifier they are conveyed by vacuum, air, or mechanically to a holding bin for packaging in bags, fiber packs, or gaylords, or transferred to tank truck or rail cars. Depending on the throughput rates, gaylords can be filled directly from the classifiers.

The final product is tested and certified to meet product specifications before shipment. If the plant has a statistical process control program, product can be certified based on the control charts and minimal physical property measurements, such as melt flow index and color.

An underwater or die face pelletizer can replace the water bath, air knife, and pelletizer. An underwater pelletizer is typically used with higher throughput operations with resins that process below 625 F (329 C). This eliminates handling many molten strands and the potential for broken strands to interfere with production. Extrusion processes running at many 1000 lb/h have greatly improved efficiency when they use underwater pelletizers. Visual pellet inspection identifies the process used to produce them. Round cylindrical pellets are produced using a water bath or water slide technology (discussed later), while a tear drop shaped pellet is manufactured with an underwater pelletizer.

The die in an underwater pelletizer is round, with die holes arranged in a circular pattern. Rotating knives fit flush against the die face in a water chamber; as the extrudate passes though the die, it is cut by the rotating knives underwater. Water cools the pellets, forming a thick skin, as the rotating blade slices the molten polymer flush with the die face. The water flow carries the pellets to a centrifugal dryer where the water is removed and the pellets are dried. The entire process is a self-contained operation; pellets exiting the dryer are ready for packaging. Figure 44.7 shows an extruder equipped with an automatic screen changer, gear pump, and underwater pelletizer [2]. An eight-blade cutter head is shown on the cutting unit. In operation, the cutting unit is moved forward on rails into the die, ensuring proper alignment between the die face and the cutter blades. Water flows into the cutting chamber bottom and out of the top. The water cools the cut pellets and conveys them away from the die. A sight glass in the exit pipe allows viewing of the cut pellets as they are transported away from the die. The key to underwater pelletizing is to have molten polymer on one side of the die and water on the other without cooling the die enough to freeze off the die holes. This is accomplished by a high flow rate through the die.

In operation, the die is bolted directly to a diverter valve; polymer flow is established through the die, and the cutter is engaged. The extruder is started with the polymer flow running to the floor through the diverter valve. Once adequate polymer flow is established, the diverter valve is closed and polymer is purged through the die with the cutter pulled back, allowing the polymer to exit as spaghetti-type strands through the die. This establishes that the die is not frozen off and that polymer is flowing freely. The diverter valve is opened, the cutter head is engaged with the die, and the cutter is rapidly brought up to cutting speed. The diverter valve is closed and water is started. The polymer flows through the die into the moving water, where it is cut into pellets by the rotating cutter head. Logic for this sequence is programmed into the underwater pelletizer, requiring the pushing of a single button to establish the proper sequence of the diverter valve closing, water flow starting, and the cutter head rotating at the proper speed. Pellets are conveyed in the water to the dryer, where the water and pellets are separated. Initially, the pellets may be oversized, as the cutter head is coming up to speed. The oversized pellets are separated from the standard product through a diverter chute. Water is recirculated through the underwater pelletizer while the pellets are dried and packaged. Water temperature and die throughput are critical to the operation to prevent polymer from freezing off in the die. If some of the die holes freeze off, due to insufficient polymer flow, the extruder throughput needs to be increased or the number of die holes decreased. Die hole freeze-off results in slightly large pellets, as a higher throughput per hour per hole is occurring through the other die holes at the same rotor speed. Cutter head speed dictates the pellet size. A slower speed with fewer blades results in larger pellets at the same overall throughput. Micropellets are made with an underwater pelletizer using smaller die holes and more cutter blades.

Before any cutting occurs, the cutter blades have to fit flush against the die face. Blades are run for a short time with no polymer present to confirm that the blades sit flat against the die face. The die face must be very smooth with round holes containing no damage or deformation around the hole exit. Defects in any die hole can lead to tails on the pellets, as the polymer is pulled away instead of being cut clean.

A third pelletizing system, supplied by Beringer Division of John Brown Plastic Machinery, is a water ring pelletizer, which is similar to underwater pelletizing in that the molten polymer is cut at the die and submerged immediately in water, creating a slurry [3]. Figure 44.8 shows the water ring process. Polymer exiting the extruder passes into a pelletizing chamber, where the polymer flows through an annular die equipped with a flexible blade that cuts the molten polymer as it exits the die. After cutting, molten pellets are thrown into a ring of falling water in the cooling chamber, where the polymer solidifies. The cooling chamber shape and high water flow prevent the molten pellets from agglomerating before cooling. Based on the die plate and pellet requirements, pellets can be formed as cylinders, spheres, or lens shaped. The pellet/water slurry is transported to a dewatering unit where excess water is removed from the pellets. Final drying is done with a centrifugal dryer, similar in operation to that used with an underwater pelletizing system. The water is recycled back to the pelletizing head and cooling chamber. Water temperature is maintained via a chiller or by replacing hot water with cold water.

A fourth cooling system is a water slide, which uses a typical strand die. The strands from the die are fed to an inclined water slide. At the end of the slide, the strands with water go into the pelletizer to be cut. The pellets and water from the cutting chamber are transported to a dewatering unit and centrifugal-type dryer for drying. Figure 44.9 shows the water slide pelletizer process.

The disposal of municipal wastes is a world problem due to their partial biodegradability. Historically, these wastes have been landfilled like TDFs. However, with the increase in municipal waste production combined with the decrease in available landfill sites, economic alternatives are being sought. One of these methods is combustion of the municipal waste, especially waste plastics. Waste plastics are one of the most promising resources for fuel use as a blending fuel because of their high heat of combustion and increasing availability in local communities. Unlike paper and wood, plastics do not absorb much moisture, and the water content of plastics is lower than the water content of biomasses such as crops or kitchen wastes.

The methods for conversion of waste plastics into fuel depend on the types of plastics to be targeted and the properties of other wastes that might be used in the process. In general, the conversion of waste plastic into fuel requires feedstocks that are nonhazardous and combustible. In particular, each type of waste plastic conversion method has its own suitable feedstock. The composition of the plastics used as feedstock may be very different, and some plastic articles might contain undesirable substances (e.g., additives such as flame-retardants containing bromine and antimony compounds, or plastics containing nitrogen, halogens, sulfur, or any other hazardous substances) that pose potential risks to humans and to the environment.

The types of plastics and their compositions will condition the conversion process and will determine the pretreatment requirements and the combustion temperature for the conversion, as well as the energy consumption required, the fuel quality output, the flue gas composition (e.g., formation of hazardous flue gases such as NOx and HCl), the flyash and bottom ash composition, and the potential of chemical corrosion of the equipment.

Smooth feeding to conversion equipment: Prior to their conversion into fuel resources, waste plastics are subject to various methods of pretreatment to facilitate the smooth and efficient treatment during the subsequent conversion process. Depending on their structures (e.g., rigid, films, sheets, or expanded (foamed) material), the pretreatment equipment used for each type of plastic (crushing or shredding) is often different.

Effective conversion into fuel products: In solid fuel production, thermoplastics act as binders that form pellets or briquettes by melting and adhering to other nonmelting substances such as paper, wood, and thermosetting plastics. Although wood materials are formed into pellets using a pelletizer, mixing plastics with wood or paper complicates the pellet preparation process. Suitable heating is required to produce pellets from thermoplastics and other combustible waste.

Well-controlled combustion and clean flue gas in fuel user facilities: It is important to match the fuel type and its quality to the burner in order to improve heat recovery efficiency. Contamination by nitrogen, chlorine, and inorganic species, for instance, can affect the flue gas composition and the amount of ash produced. Ash quality must also be in compliance with local regulations when disposed of at the landfill. Therefore, the fuel quality must be controlled in order to minimize its environmental impact.

Waste plastics are composed primarily of low-density polyethylene and high-density polyethylene products. These two types make up nearly 60% of all plastic production. Figure 5.5 denotes the distribution of waste plastics by type [57]. Table 5.17 classifies various plastics according to the types of fuel they can produce. As noted, thermoplastics consisting of carbon and hydrogen are the most important feedstock for solid fuel production.

Because plastics are made from hydrocarbons, they tend to have very high energy values, as noted in Table 5.18. These values compare well with fuel oil and are considerably higher than most coals and TDF [57, 58].

Refuse-derived paper and plastics densified fuel (RPF) is prepared from used paper, waste plastics, and other dry feedstocks [59]. Within the plastics, the thermoplastics play a key role as a binder in the briquetting or pelletizing process. Other components such as thermosetting plastics and other combustible wastes cannot form pellets or briquettes without a binding component. Approximately 15 wt% thermoplastics is the minimum required to be used as a binder to solidify the other components; however, more than 50 wt% could cause a failure in the pellet preparation. The components of RPFs are mainly sorted from industrial wastes and are sometimes also obtained from well-separated municipal waste. This type of solid fuel was set to be standardized in the Japanese Industrial Standards (JIS) in April 2002.

The plastic contents can be varied (within a range) to meet the needs of fuel users. The shape of the fuel will vary according to the production equipment (e.g., a screw extruder is often used to create cylindrical-shaped fuel with a variable diameter and length). In the production of solid fuel, the contamination of the targeted plastics with other plastics containing nitrogen, halogens (Cl, Br, F), sulfur, and other hazardous substances may cause air and soil pollution through flue gas emissions and ash disposal (e.g., inorganic components such as aluminum in the multilayer film of food packages produces flyash and bottom ash). Other contaminants such as hydrogen chloride might cause serious corrosion damage to the boiler.

Examples of these two types of production systems are presented. The first is a large-scale system with pretreatment for the separation of undesirable contamination such as metals and plastics containing chlorine; the other is a small-scale model without pretreatment equipment.

Industrial waste plastics, which have been separated and collected in factories, are ideal for use in solid fuel production. The fuel production facility consists of a waste unloading area, stockyard, pretreatment equipment, pelletizing equipment, and solid fuel storage. The pretreatment process includes crushing and sorting to remove unsuitable materials from the incoming wastes. A schematic diagram of the pretreatment process is shown in Figure 5.6.

After pretreatment, the mixture of paper and plastics is further processed in a secondary crusher and sorting process (conveyor and magnetic separator), and the resulting mixture is pelletized to produce solid fuel, as shown in Figure 5.7. The solid fuel is cooled to prevent natural ignition during storage, and it is further stored for shipping. The output of the process is usually solid fuel pellets of dimensions between 6 and 60 mm in diameter and 10 and 100 mm in length. The heating value of the pellets will change depending on the content of the plastics. A mixture of paper and plastics of a 1:1 weight ratio gives a heating value of approximately 7000 kcal/kg or higher.

The second system has a solid fuel production of 150-kg/h capacity. This facility does not have a pretreatment process, so the combustible wood, paper, and plastic wastes are directly fed into the crusher of the facility. This is carried out using a handling machine, as shown in Figure 5.8, where an operator must control the feed into the crusher to maintain a suitable ratio of each type of waste in order to produce the required fuel qualities, such as heating value. After crushing, the materials are transported through a pipe conveyor and are introduced into a twin-screw pelletizer. Figure 5.9 shows the entire process (the crusher, the pipe conveyor, and the pelletizer).

Experts agree that properly equipped, operated, and maintained incinerators or combustion facilities can meet the latest U.S. emissions standards while cofiring waste plastics [60]. In fact, plastics can be successfully burned in dedicated energy recovery facilities that achieve high combustion temperatures to eliminate dioxin and furan production.

One concern in utilizing waste plastics is in preventing melting of the material rather than combusting the material. If a melted plastic reaches a tube surface, it can stick and reduce heat transfer to the steam. In addition, the sticky surface permits solid ash particles to agglomerate and increase deposition [61].

Depending on how the torrefaction system is designed, will dictate its energy requirements. Chapters 6 and 7 have shown torrefaction systems that are involved as stand-alone systems as well as several cogeneration processes. We will focus initially on stand-alone systems and then discuss the considerations of cogeneration systems.

The stand-alone torrefaction systems generally requires four main components (1) a feedstock handling system, (2) a reactor, (3) torr-gas capture, and (4) torrefied biomass postprocessor. Practically all the power requirements for components 1, 3, and 4 will be satisfied with electric motors operating the conveyance systems for introducing the feedstock in the reactor and conveying the torrefied biomass out of the reactor and into the pelletizer, torr-gas compressors, and electronic control systems. Efficiency gains may be realized by optimizing the energy requirements of each operation. While the reactor is expected to have electronic controls for controlling reaction operating conditions, the primary energy consumer is the thermal energy provided to drive the reaction. The thermal energy may be provided by electrical resistive heaters, burning fuel from an external source (natural gas, coal, etc.), burning some or all the torr-gas produced from the reactor, or utilizing waste heat from a cogeneration process. Each heat source has its own benefits and drawbacks from financial and environmental perspectives. Electricity for heat is an option that has no emissions on site, but is quite expensive per Btu of thermal energy. In contrast, burning natural gas or propane is much less expensive per unit of thermal energy and is rather clean burning but still a carbon-based fossil fuel. However, it may not make much financial sense to have large-scale torrefaction devices use rather high value liquid fuels in the process. Perhaps more economically viable options for the reaction are to use waste heat from a cogeneration process or use the torr-gas to heat the reactor. Exhaust from a natural gas-fired power plant would be at temperatures sufficient for torrefaction and may be underutilized for other processes.

Nanocomposites represent an alternative approach for improving the properties of a biopolymer. Many researchers have examined the potential for nanoparticles reinforcement of biopolymers, with mixed degrees of success [611]. Biopolymer-based nanocomposites have been reported to have improved physical, mechanical, and thermal properties, including barrier properties, tensile strength, and thermal stability, respectively [12]. Substantial savings to the costs and the weight of the materials can also be achieved. It has been estimated that a nanocomposite with a nanofiller content of 4wt% provides equivalent mechanical properties compared with a conventional microcomposite with a microfiller content of 20wt% [13]. However, the polymer nanocomposites did not live up to the expectations because they did not provide substantial improvements to the mechanical properties of materials in comparison to the conventional microcomposites. The main reason the mechanical properties of polymer nanocomposites still fall short of their theoretically predicted values is the inherent tendency of the nanoparticles to aggregate and/or agglomerate, and their inability to disperse homogeneously within the polymer matrix.

Most inorganic nanofillers, such as clay nanoparticles, are hydrophilic, whereas the major aliphatic polyesters, in particular PLA, are hydrophobic. Therefore, most inorganic nanofillers are surface treated to render them hydrophobic e.g., fumed silica treated with silicone oil or hexamethyl disiloxane (e.g., Aerosil, Evonik Industries AG). Numerous efforts have been made to solve the previously mentioned problems. A variety of nanofiller dispersion methods, such as in situ polymerization [6], solution intercalation method in N-dimethylacetamide [7], and melt intercalation technique using modified montmorillonite [6,14], have been applied. Biodegradable nanocomposites were prepared on the basis of PLA and montmorillonite clay with stacked intercalated and partially exfoliated morphologies [8].

KR100655914 B1 (2006, SK NETWORKS CO LTD) discloses a biodegradable nanocomposite comprising the following: 7095wt% of an aliphatic polyester and 530wt% of a powder of layered inorganic silicate comprising muscovite and/or phlogopite dispersed in oil. The dispersion of the powder of layered inorganic silicate in oil improves dispersibility with the biodegradable polymer and enables penetration of the oil into the pores between the layers of the powder of layered inorganic silicate, thereby further expanding the pores or interlayer spacing. The aliphatic polyester is selected from PLA, poly(glycolic acid) (PGA), PCL, and P3HB; preferably, PLA is used. The method for preparing the biodegradable nanocomposite is outlined in Figure 12.2.

KR20060003580 A (2006, TORAY SAEHAN INC) discloses a biodegradable polyester composition comprising 4095wt% PLA, 560wt% aliphatic-aromatic polyester, and 0.110wt% inorganic nanoparticles modified with various intercalating agents. Suitable inorganic nanoparticles are montmorillonite, hectorite, saponite, atapulgite, sepiorite, and vermiculite. The intercalating agents are selected from alkylene glycols, such as ethylene glycol, propylene glycol, and tetramethylene glycol.

WO03022927 A1 (2003, UNITIKA LTD) discloses a biodegradable polymer composition for molding comprising 100pbw of a biodegradable polyester containing at least 50pbw of PLA having a Tm160, and an MFI of 0.150g/10min under 21.2 load, and 0.120pbw of a phyllosilicate containing primary-tertiary amine salt, quaternary ammonium salt, or phosphonium salt bonded as ions. For improvement of the dispersibility of the phyllosilicate in the biodegradable polyester, at least 0.110pbw of a compound selected from a polyalkylene oxide, an aliphatic polyester, a polyalcohol ester, and a polycarboxylic acid ester having an affinity for both the biodegradable polyester and the phyllosilicate having a boiling point of at least 250C, and a Mn of 20050,000 may be added as a compatibilizer. The phyllosilicate is preferably dispersed in the biodegradable polyester in a completely exfoliated state in which the layers of the phyllosilicate are exfoliated from each other, in an intercalated state in which molecules of the polymer are intercalated between the layers, or in a mixed state in which the exfoliated state and the intercalated state are present. From a quantitative point of view, the average thickness of single and multiple layers of the phyllosilicate is preferably 1100nm, more preferably 150nm, and most preferably 120nm. The interlayer distance of the phyllosilicate is preferably 2.5nm or higher.

WO2007022080 A2 (2007, UNIV MICHIGAN STATE) discloses a biopolymer composition of nanocomposite structure comprising three materials: (1) a bio-based biopolymer, such as PLA or P3HB; (2) a fossil fuelderived biopolymer, such as PBAT; and (3) a fatty acid triglyceride quaternary ammonium salt modified nanoclay to develop a high-barrier, biodegradable material for packaging. An exemplary nanocomposite is formed by melt compounding particularly by extrusion, poly(l-lactic acid) (PLLA) with (PBAT) with Cloisite 25A/30B (injection-molded rigid samples).

CN101469072 A (2009, SHENZHEN ECOMANN BIOTECHNOLOGY CO LTD) discloses a melting intercalation method for the preparation of a PHA/montmorillonite nanocomposite comprising the following steps: (1) preparing a graft polymer of PHA through an initiator under the condition of melting, (2) preparing a masterbatch comprising the grafted PHA as matrix and organically modified montmorillonite as a filling component, and (3) melting and blending the masterbatch and PHA to prepare the composite material. The interval of a montmorillonite layer of the prepared composite material is 15nm. The montmorillonite has the advantage of dispersion evenness, good material mechanical property, better heat resistance, and film-forming property.

US2012289618 A1 (2012, KOREA INST SCI & TECH) discloses a method of preparing a biodegradable nanocomposite, including the following steps: (1) putting two kinds of single-phase biodegradable polymers (namely, poly(d-lactic acid) (PDLA) and PLLA), a clay, and a small amount of organic solvent into a reactor; (2) injecting a supercritical fluid into the reactor and applying a predetermined temperature and pressure; (3) uniformly mixing the single-phase polymers and clay to form a stereocomplex (or stereoisomeric) composite and causing a dispersion reaction of the clay; and (4) collecting the PLA/clay nanocomposite. The nanosized clay used as a filler is preferably a clay mineral having a layered structure in which oxide layers having a negative charge are laminated to one another, and may be a natural clay or synthetic clay having a thickness of approximately 1nm, a length of approximately 2180, and an aspect ratio of approximately 2000 for each layer. More specifically, the clay compound may be a phyllosilicate having a negative charge made of aluminum silicate or magnesium silicate layers, or potassium or sodium phyllosilicates filled with sodium ions (Na+) or potassium ions (K+) between phyllosilicate layers. The phyllosilicates are preferably selected from montmorillonite, hectorite, saponite, beidellite, nontronite, vermiculite, volkonskoite, sauconite, fluorohectorite, magadite, kaolinite, and halloysite. The PLA/clay nanocomposite has the form of a particle or porous foam (see Chapter 5: Compounding; Section 5.3.1: With Inorganic Compounds).

WO2013005914 A1 (2013, GLOTECH CO LTD) discloses the molding of a bone-fixing composite material useful for repairing osseous fracture in human skull. The molding is obtained by: (1) mixing nanocarbon particles whose end is substituted by amine or amide, and a biodegradable biopolymer at constant temperature, in an extruder; (2) heat kneading; (3) extruding through stretching; (4) continuously cooling; and (5) injection molding into desired shape. The biodegradable polymer is selected from PGA, PDLA, PLLA, PCL, polyesteramide, polyoxalate, aliphatic polycarbonate, poly(glutamic-co-leucine), and their copolymers. The amount of nanocarbon granules is 0.110wt% with respect to biodegradable organic polymer. The composite is claimed to have excellent dispersibility, biodegradability, biocompatibility, impact strength, and tissue affinity, improves mechanical strength and physicochemical properties of the bone, and prevents adverse effects, such as corrosion of bone by acid and inflammation.

CN102167894 A (2011, CHANGCHUN APPLIED CHEMISTRY) discloses a PLA/graphene nanocomposite preparation method thereof, wherein the graphene oxide accounts for 0.15wt% of the PLA (see Chapter 5: Compounding; Section 5.4: Making Masterbatches). The prepared PLA/graphene nanocomposite had a tensile strength up to 79MPa, a tensile elastic modulus up to 3100MPa, a notch impact strength up to 12kJ/m2, and a HDT up to 95C.

CN103030791 A (2013, UNIV HEBEI TECHNOLOGY) discloses a method of fabricating a PLA/nanodiamond composite with a solvent backflow method comprising the following steps: (1) add a PLA aqueous solution and nanodiamond to a reactor; (2) ultrasonic dispersing the mixture at room temperature for 1h; (3) add a catalyst and an organic solvent and stir at 150C for 1248h; (4) remove the solvent and water generated by reaction by reduced pressure distillation; and (5) dissolve the reacted product with acetone, precipitate with distilled water, and dry to obtain the PLA/nanodiamond composite. The nanocomposite comprises the various components in the following proportions: 10wt% of PLA aqueous solution, 0.010.50wt% of nanodiamond, 0.0350.06wt% of catalyst (e.g., stannous octoate), and 1015wt% of solvent (e.g., toluene, xylene, or diphenyl ether). The solvent backflow method used is lower in reaction temperature, simple in equipment, easy and simple to operate, and easy to implement. The thermostability of PLA is improved substantially. Both the 5% weight loss temperature and the 10% weight loss temperature of the composite increase approximately by 40C, whereas the complete decomposition temperature is increased by 60C.

Most of the natural polymers, such as starch, cellulose, chitin, lignin, and keratin, are polar, and thus compatible with the natural clay. Yet, the incorporation of the natural clay particles on these biopolymers to produce nanocomposites does not provide satisfactory properties because the main matrix is moisture sensitive and the melt strength of the resulting product becomes poor at high clay loading, making the extrusion process and blown molding difficult.

DE19504899 A1 (1996, NAT INST RES INORGANIC MAT) discloses a method for the production of a porous body of polysaccharide/clay composite by: (1) rapidly freezing an aqueous solution of starch, Na alginate, carboxymethylcellulose (CMC), or a derivative thereof, and a clay solution; and (2) vacuum drying the frozen product without melting the ice. The clay has a size of at most 2 m, and is selected from montmorillonite, saponite, beidellite, kaolinite, allophane, or bentonite, or a synthetic clay. The polysaccharide/clay composite can be used as a shock-absorbing, heat insulator, or sound-absorbing material. The presence of clay provides high compression strength when compared with other commercially available foamed polymers.

EP1134258 A1 (2001, TNO) discloses a biodegradable nanocomposite comprising a natural polymer, a clay, and, optionally, a plasticizer. The natural polymer is selected from starch, cellulose, chitosan, alginic acid, inulin, pectin, and derivatives and combinations thereof. The clay has a cation exchange capacity of 50200meq/100g, and is preferably a smectite-like clay mineral. The nanocomposite is obtained by preparing a suspension of the clay in water and extruding the suspension together with the natural polymer and the plasticizer at elevated temperature (35200C). The clay suspension is ion exchanged with a modifying agent, the modifying agent being a surfactant having from 6 to 16 carbon atoms, a functionality compatible with the natural polymer and an ammonium, phosphonium, or sulfonium group. However, this nanocomposite generally has a poor melt strength to be used in blown film applications, and because it has a natural polymer matrix, it is permeable to moisture and soluble in water, which decrease their shelf life and makes the composite film unsuitable for packaging purposes (2007, EP1860138 A1, 2007, SABANCI UNIVERSITESI).

Melt blending the natural polymer/clay nanocomposite in an extruder at a suitable temperature of 110250C with a synthetic polymer selected from polyethylene oxide, low-density polyethylene, high-density polyethylene, polypropylene, and the combination thereof, and any polyolefin having a Tm lower than the degradation temperature of the natural polymer.

The method comprises an optional step of adding 2580wt% plasticizer on the basis of the weight of the natural polymer. The clay is selected among naturally occurring smectite clays having a layered structure and a cation exchange capacity of 30250mEq/100g. The weight ratio of the amount of clay to the amount of polymeric matrix is 110wt%, preferably 15wt%.

Except the inorganic nanofillers/nanofibers, there are also reports of using cellulose nanowhiskers for the reinforcement of PLA [15]. The cellulose nanowhiskers were pretreated with poly(vinyl alcohol) (PVOH) for improving the dispersion of the nanofibers in the matrix. When an extruder was fed with a mixture of nanofibers and PVOH in the form of both spray-dried powders and suspension, a phase separation occurred consisting of a PVOH phase, in which most of the nanofibers were concentrated, and a PLA phase.

Petersson etal. [16] describe a method for the production of nanocomposites on the basis of a PLA matrix reinforced with cellulose nanowhiskers by means of casting. The nanofibers were subjected to a treatment with tert-butanol or with a surfactant to disperse them in the solvent, and they were subsequently incorporated into the PLA matrix through the technique of casting using chloroform as a solvent. However, it was not possible to completely prevent the agglomeration of the crystals (2011, WO2011138485 A1, CONSEJO SUPERIOR INVESTIGACION).

Oksman etal. [17] describe a method for producing a reinforced polymer comprising PLA and a microcrystalline cellulose reinforcing material. The reinforcement material was subjected to a treatment with N,N-dimethylacetamide (DMAc) and lithium chloride (LiCl) to partially separate the cellulose nanofibers. The suspension of nanofibers was mixed with the polymeric matrix in an extruder using the technique of melt mixing. The treatment with DMAc/LiCl causes the degradation of nanocomposites at high temperatures. In addition, the dispersion of the nanofibers is not complete and, therefore, there was not a considerable improvement in the mechanical properties (2008, US2008108772 A1, NTNU TECHNOLOGY TRANSFER AS).

Grunert etal. [18] describe a method for the incorporation of bacterial cellulose nanofibrils in a cellulose acetate butyrate matrix (CAB) by means of casting. The cellulose nanofibers showed a tendency to agglomerate. By means of chemical modification of the surface of the nanofibers (trimethylsilylation), the dispersion was improved. However, the chemically modified nanofibers had worse reinforcement properties (2011, WO2011138485 A1, CONSEJO SUPERIOR INVESTIGACION).

US2008108772 A1 (2008, NTNU TECHNOLOGY TRANSFER AS) discloses a method for producing a nanocomposite by mixing a dispersion comprising a plasticizer and cellulose nanowhiskers into CAB. The nanowhiskers dispersion is pumped into an extruder together with CAB. The extrusion process will thoroughly mix the cellulose whiskers into the CAB matrix, thus providing a homogeneous mixture being highly reinforced.

WO2011138485 A (2011, CONSEJO SUPERIOR INVESTIGACION) discloses a method for producing a nanocomposite comprising the following steps: (1) mixing of a nanoreinforcement with a polymeric matrix in liquid state; (2) electrospinning of the dispersion obtained in (1); and (3) melt mixing of the product obtained in step (2) with a polymeric matrix equal to or different from the one used in step (1). The polymer matrix can be a biodegradable polymer. The nanoreinforcement is selected from spherical, fibrillar, tubular, lamellar nanostructures or any of their combinations. In a preferred embodiment, the fibrillar nanostructure is made of cellulose.

Plant cellulose nanofibers extracted from highly purified cellulose were lyophilized and dispersed in water by means of the application of ultrasound. They were then centrifuged at 12,500rpm, 15C and 20min, the water was removed from the supernatant by means of decanting, and the water was replaced by acetone, which was replaced later using the same method by chloroform (solvent used for PLA). This cycle was repeated four times to ensure the complete substitution of the solvent and, therefore, obtaining plant cellulose nanofibers disperse in the non-polar solvent chloroform. The solution of chloroform with cellulose nanofibers was used to dissolve the PLA pellets, so the final concentration of nanofibers with respect to the weight of PLA in the solution was set at 8%. To improve the electrospinning of the matrices, 20% polyethylene glycol and 80% PLA were added, such that both materials represent 56wt% of the chloroform. The solution is introduced in 5-mL glass syringes connected through Teflon tubes to several 0.9-mm-diameter stainless steel needles. The needles are connected to an electrode that, in turn, is connected to a power source of 030kV. A voltage of 12kV is applied, and the solution is pumped through said needles with a flow of 0.6mL/h. The counter electrode is connected to a plate (collector) covered with aluminum foil, where the electrospun structures are collected, being the distance between needle and plate of approximately 12cm. The process is performed at room temperature. In this way, PLA electrospun structures that contain disperse plant cellulose nanofibers are obtained.

This aggregate is produced from PFA which is a powder by-product of pulverized bituminous coal used to fire the furnaces of power stations. Suitable PFA of not more than about 810 per cent loss on ignition which results from unburnt carbon in the form of coke (char), is first homogenized in bulk in its powder form. Once homogenized, it is then conditioned through a continuous mixer with about 1215 per cent of water and, as necessary, an amount of fine coal is added to make up the fuel content to about 10 per cent of the dry mass of the pellet to enable it to be fired. This conditioned mixture of PFA is then fed at a controlled rate onto inclined and revolving pelletizing dishes. The size and degree of compaction of the formed green pellets depend on the inclination and speed of rotation of the dish, the rate of addition of the conditioned PFA as well as a further amount of water spray. The formed pellets discharge from the pelletizer at a diameter of about 1214mm. Without any further treatment these pellets are conveyed to the sinter strand where they are fed by spreading to form an open-textured and permeable bed to the width and depth of the sinter strand. The sinter strand is a continuously moving conveyor comprising a series of segmented and jointed grates through which combustion air can be drawn to fire the pellets and combustion gases exhausted. Once on the bed, the sinter strand immediately carries the pellets under the ignition hood that fires the intermixed fuel. The chemical composition of PFA resembles that of clay but, unlike clay, as the PFA has already been fired, no pre-drying or pre-heating of the pellets is necessary, as the pellet is able to expire the water as vapour and combustion gases without incurring damage to the particles. Once ignited at about 1100C, and as the bed moves forward, air for combustion is drawn by suction fans beneath the grate. The process is controlled to prevent the particles of PFA becoming fully molten so that (a) they coagulate sufficiently to form an aggregate and (b) the aggregate particles are only lightly bonded to each other. The correct amount of coagulation within the pellets is obtained by varying both the speed of the sinter strand and the amount of air drawn through the bed of pellets.

The finished product on the sinter strand is a block of hard brick-like spherical nodules, lightly bonded by fusion at their points of contact. As the sinter strand reaches the end of its travel and commences its return to the feeding station a segment of the bed of the finished product is discharged into a breaker. This action separates the aggregate pellets prior to grading.

While the surface and internal structure of the finished pellet (Figure7.4) is essentially closed it contains encapsulated interstices between the coagulated PFA particles. While these interstices are minute they are penetrable by about 20 per cent moisture but eventually breathe sufficiently to allow any water to evaporate even when encased in concrete.

effects of granulator structure and cooperating mode with slag tube on the centrifugal granulation characteristics of molten slag - sciencedirect

Molten slag flows to the edge of the rotating cup and is rapidly broken into droplets under the centrifugal force provided by the high-speed rotating cup. The diameter of the droplets is closely related to subsequent waste heat recovery and resource utilization. To clarify the law of centrifugal granulation of molten slag, the effects of the structure parameters of the rotating cup, the way the slag tube and the rotating cup are matched, and the eccentricity on characteristics of molten slag are investigated in the paper. In the present work, an unsteady, three dimensional, and two-phase model is established to analyze the influence of the above factors on the particle size distribution, average particle size, and maximum and minimum particle size of the granulated slags. As a result, in the range of 045, the increase of the inclination of the inner wall surface of the rotating cup can strengthen the granulation effect. To ensure the granulation effect, the ratio of the diameter of the slag tube to the rotating cup should not exceed 0.44, and the installation height of the slag tube should be as close as possible to the granulator. Moreover, to make the particle size uniform after granulation, the slag eccentricity is kept within 5%. Finally, the dimensionless correlation of the diameter of droplets was obtained to guide industrial practice to control the particle size of the granulated slag particles in the paper.

experimental investigation of hydrocarbon contamination at the headdisk interface | springerlink

Hydrocarbon oil contamination of the headdisk interface is investigated. Optical surface analysis, atomic force microscopy, and contact angle measurements are used to study the adsorption characteristics of hydrocarbon contaminants on the disk surface. Optical microscopy, scanning electron microcopy, energy-dispersive X-ray spectroscopy, and time-of-flight secondary ion mass spectrometry are used to investigate hydrocarbon contamination at the headdisk interface. Temperature and time were found to significantly influence hydrocarbon contamination. The results agree well with molecular dynamics simulation studies.

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We would like to acknowledge Western Digital Corporation for an internship provided to Young Woo Seo and for allowing the use of their facilities to perform a number of the tests reported in this study. Also, we would like to acknowledge Dr. Raj Thangaraj, Dr. Jih-Ping Peng, Dr. Min Yang, and Dr. Joe Hanke for their insights and helpful discussions.

As shown in Fig.18, the UAM approach models CH2 and CH3 as beads in order to reduce computational cost while accurately reproducing thermodynamic properties of linear hydrocarbon chains [28]. Using the Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) [34] and appropriate potential functions and corresponding parameters [2833], we were able to simulate the crystallization of linear hydrocarbon chains.

The linear hydrocarbon chains were first positioned as shown in Fig.8a. Then, we imported the position data and applied the above potential functions in LAMMPS [2834]. The MD simulation was first carried out in the canonical (NVT) ensemble with a time step size t=2fs. An NVT ensemble is a thermo-statistical system in which the number of atoms (N), the volume of the simulation box (V), and the temperature (T) are kept constant. With the NVT ensemble, we randomly distributed the linear hydrocarbon chains for 100,000 time steps (Fig.8b). The system was then equilibrated at 450K for a total of 600,000 time steps. Thereafter, we used the microcanonical ensemble (NVE) and the Langevin thermostat to quench the equilibrated system of linear hydrocarbon chains to 290K for 1,000,000 time steps, at a cooling rate of \(1.5 \times 10^{11} \,{\text{K}}/{\text{s}}\). The system was then equilibrated at 300K for 50,000,000 time steps in order to grow crystals of linear hydrocarbon chains as shown in Fig.8c.

Seo, Y.W., Ovcharenko, A., Bilich, D. et al. Experimental Investigation of Hydrocarbon Contamination at the HeadDisk Interface. Tribol Lett 65, 54 (2017).

a review of process intensification applied to solids handling - sciencedirect

A number of solid handling applications where PI has been involved are reviewed.Criticization/precipitation are the most studied applications for solid handling.Hybrid and alternative energy technologies are identified for further development.

Process intensification (PI) is a strategy aimed at transforming conventional chemical processes into more economical, productive and green processes. Its fundamental concept hinges upon the volume reduction of processing equipment resulting in enhanced mixing and heat/mass transfer as well as a multitude of other benefits. To date, the focus of PI has been on processes mainly involving gas/liquid systems. Solids handling applications have been more limited as fouling and blockages can occur due to large concentrations of solids in smaller equipment sizes. Appropriately designed equipment is therefore a key consideration for intensifying industrially-relevant solids handling processes.

In this review paper, we highlight a number of solid processing applications including precipitation, separation, granulation and milling, etc. where PI has been demonstrated. Much effort has been directed at reactive crystallization and precipitation in various intensified technologies, exploiting their enhanced mixing capabilities to produce uniformly distributed nano-particles. Generally, the objective in many of these processes has focused on transforming solids handling in batch processes into continuous ones with processing time reduction and improved energy efficiency. The review highlights the considerable opportunity for further development of multifunctional technologies in solids handling applications such as granulation and drying, the subject of a European Commission-funded HORIZON 2020 project.


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bulk disc resonators radial and wineglass mode resonance characterization for mass sensing applications | springerlink

This paper reports the design and fabrication of bulk mode micromechanical disc resonators operating in radial and wine-glass modes of excitation. The reported structures are fabricated utilizing a single crystal SOI wafer through micromachining processes. Both resonators are fabricated on a device layer with a thickness of 20m and a gap size of 1.75m between the resonant beam and surrounding electrodes. Four anchors support the resonant disc using a T-shaped connection stem. The designed structures resonate at 2.87 and 3.99MHz, in wine glass and radial modes respectively, and are electrostatically actuated by a DC voltage of 110V between the disc and electrodes. The designed resonators show high quality factors while operating in air, 1,1876.2 for wine-glass and 7380 for radial. In addition, the resonators are used for distributed and point mass measurements of a sputtered gold metal layer. The wine glass resonator shows a frequency down shift of 1kHz for a 44 ngr gold point mass, and a frequency shift of 22kHz for a distributed mass of 83 gr. Same test is performed on radial mode resonator and a resonance frequency shift of 1.24 and 25.54kHz was observed for point and distributed mass, respectively in air and at room temperature.

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This work was fully implemented in the Nano-Fab at the University of Alberta and was partially supported by CMC Canada. The authors would like to thank the Nano-fab staff at the university of Alberta for their collaboration.

Zarifi, M.H., Daneshmand, M. Bulk disc resonators radial and wineglass mode resonance characterization for mass sensing applications. Microsyst Technol 22, 10131020 (2016).