Handling of Hollow Glass Microspheres

Handling of Hollow Glass Microspheres

10 Handling of Hollow Glass Microspheres Baris Yalcin Handling of powders and bulk solids has been reported in numerous references in the literature...

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10 Handling of Hollow Glass Microspheres Baris Yalcin

Handling of powders and bulk solids has been reported in numerous references in the literature. Characterization of powders and bulk solids regarding their flow properties plays an important role, for example, for product development and optimization and customer support. The discharge of powders and bulk solids from silos, hoppers, transport containers, etc., may result in severe problems due to flow obstructions, segregation, shocks and vibrations, or unsteady flow. To avoid such complications, solutions have to be found considering the flow properties of the bulk solid. Hollow glass microspheres (HGMs) share several of the common handling techniques and equipments applicable to other bulk solids and powders. However, due to their low density, fragile, and free-flowing nature, some differences exist between HGMs and other bulk solids such as talc and CaCO3. It is the purpose of this chapter to review some of the equipment that has been successfully utilized in the past to transport and store HGMs. It is important to note that HGMs, depending on the manufacturer, may come with different free-flow behavior. The techniques and equipment, that is, hoppers, metering and mixing equipment, and pumps, mentioned here are based on experience with 3M HGMs and were not verified for other HGM brands by the authors of this book.

Silos and Hoppers In high-volume processing operations, day and weigh hopper combinations are recommended (Figure 10.1). While the day hopper stores enough HGMs to feed one or more weigh hoppers throughout a work period, weigh hoppers are placed close to the mixers to help minimize any delay in getting HGMs into the process. For a smaller processing operation, a single hopper is fed directly from the shipping container functioning both as a day and weigh hopper. Symmetrical hopper designs can create funnel flow and discharge problems as shown in Figure 10.2. HGMs in symmetrical hoppers could Hollow Glass Microspheres for Plastics, Elastomers, and Adhesives Compounds http://dx.doi.org/10.1016/B978-1-4557-7443-2.00010-4 Copyright © 2015 Elsevier Inc. All rights reserved.

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Figure 10.1 Day and weigh hopper combinations [1]. With permission from 3M.

Figure 10.2 Flow issues with HGMs. Courtesy of 3M.

bridge or arch, and then rat-hole when discharging. Flow could be erratic and density of flow can vary. Product can remain in dead zones until complete clean out of the system. To help avoid these problems, asymmetrical day or weigh hopper designs with steep sides and offset entrance, outlet or sloped regions, and flow inserts are recommended (Figure 10.3). Asymmetric hopper design helps improve flow speed and uniformity, while reducing the amount of dust generated with high-volume conveying equipment.

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Side view Step sided Asymmetric hopper Diaphragm Aeration system Flow insert

Top view

• Fluidize with clean dry air • Add a flow insert • Use agitaƟon • Use vibraƟon • Use a steep-sided hopper/cone

Off-center discharge

Figure 10.3 Asymmetric hopper design for HGMs. Courtesy of 3M.

If an existing symmetrical hopper is to be used, an aeration system to fluidize the HGMs can be incorporated to help mitigate these discharge problems. Fluidizing with clean dry air helps reduce discharge time and HGM breakage. Aeration also helps blend the materials, improving output consistency. Aeration increases bubble bulk volume considerably, so the hopper must be large enough to accommodate the increased volume. A combination of aeration with asymmetrical hopper design provides the most optimal discharge of HGMs from hoppers.

Transfer of HGMs Typically, boxes are emptied by vacuum suction. Vacuum can be obtained from a vacuum receiver or a double diaphragm pneumatic pump.

Double Diaphragm Pneumatic Pump Transfer of HGMs from a Box or a Bag A 3-in pneumatic double diaphragm pump is typically used to move lightweight powders. It is a lower initial cost method that effectively

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transfers aerateable low-bulk density powders. The air-driven pump is a combination of pull/push, vacuum-pressure conveying system. The pump pulls material by vacuum into its inlet, and then pushes the material along the conveying line with pressure. In the pressure conveying system, poor line connections will leak dust into the workplace. The pump should be placed closer to the process in order to pull material a longer distance. This will reduce line plugging. Adding purge air into the pump chamber when the pump is pushing material into the line helps to decrease pump plugging and stalling. The pump should not be stopped when it is full of powder. Often a vacuum relief valve is mounted close to the pump suction port. A bleed-down valve at the pump outlet is suggested for relieving pressure from a plugged line or pump. Purging the pump and the conveying system with air or other compatible gas is suggested before and after HGM transfer. The pump should be operated between 25 and 50 psi and below the hose maximum burst pressure rating (Figure 10.4). Figure 10.5 shows a double diaphragm pump vacuuming HGMs from a box to an asymmetric hopper and mixer. Site tube

Site tube

Line tap

Bleed down for relieving pressure from a plugged line or pump.

Pump Suction relief From storage

Purge air added into a pump chamber when it is pushing material into the line helps to decrease pump plugging and stalling.

Dry plant air 60 – 90 psi

Filter

Regulator

Figure 10.4 Double diaphragm pump system [2]. Reconstructed from Ref. [2] with permission from 3M.

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Line velocity 600–800 fpm <1200 fpm

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Hopper refill

Level

Pinch valves or butterfly valves or rotary valves (>6 blade) / NO more than 1200 fpm / Air assists between 1 and 3 psi

Dust collection

L-I-W hopper

Dry air

Box

Figure 10.5 Transfer of HGMs from a box using a double diaphragm pump to an asymmetric hopper followed by the mixer (L-I-W ¼ loss-inweight) [2]. Reconstructed from Ref. [2] with permission from 3M.

Vacuum Wand Pickup Aerated pickup wands are often constructed from a rigid pipe. The wand is fitted with a low-pressure compressed air line to fluidize material near the tip. The wand tip should have a protective guard to reduce plugging with the liners in the box. This enables the system to resume transfer even if the tip is covered with material. The diameter is the same as the hose connection. Fluidizing air must be synchronized with the transfer system operation. There are several commercially available wands or they can be made easily. Figure 10.6 shows a simple handmade vacuum wand, effectively used with a double diaphragm pump. Such a wand features adjustable air registers for regulating air in outer supply tube and inner fluidizing air channel. As the diaphragm pump creates vacuum to pull the HGMs, the ambient room air is sucked in through the registers of the wand into the inner channel, which aerates the HGMs in the box near the wand tip. The wand should be inserted into an HGM box at one of the top corners and slowly guided diagonally through the microspheres to the opposite bottom corner to create a trench. This will allow material to fluidize as it falls from the trench walls into a pool for pickup by the wand.

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Vacuum Air in

Bypass tube Box

Material

Tip guard To prevent pulling in liner

Figure 10.6 Vacuum wand. With permission from 3M.

Conveying Lines and Hoses Conveying lines connect the various system components for microsphere handling. Typically, a transfer system uses 3-in (76 mm) components. HGMs should be transferred with a line velocity of less than 1200 ft/min (365 m/min). Lines with long radius bends or sweeps are suggested instead of 90 elbows. Lines can be combinations of rigid and flexible materials. All conveying lines and all components should be electrically grounded. Hoses with a smooth inner bore and a conductive drain wire are suggested. The drain wire must be connected to metal connectors. Flexible lines may range from braided chemical hose, semitransparent PVC, clear polyurethane to interlocking metal hose. Caution has to be exercised as some of the hose materials are limited to use in temperatures above 20  F.

Flow Aids Experience shows that fluidizing a material makes it much easier to handle. Air assists (Figure 10.7) in the conveying lines are used to keep conveying lines trouble-free. They are typically mounted at the bottom of vertical line legs and about every 50 ft in horizontal line runs. Air pads mounted near the discharge port in hoppers are suggested to help fluidize material for easy transfer.

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Figure 10.7 Air assist (left) and air pads (right). Reconstructed from Ref. [2] with permission from 3M.

Sight Windows and Sight Tubes Sight windows and sight tubes are used to observe material flow in order to locate a problem in the transfer system. Suggested mounting locations are at the pump outlet or the bottom of vertical legs, or optionally at the receiving vessel entrance. Sight tubes use PyrexÒ glass or transparent PVC schedule 80 tubing. Grounding with a wire across the length of the sight tube is suggested. Polycarbonate material is suggested for sight windows.

Tilt Table for Boxes Large boxes are often placed with the pallet on a tilt table. The table is tilted after a portion of the material has been removed. It may include options for a wand holder and vibration. Excessive vibration will decrease glass microsphere transfer rates.

Vacuum Transfer of HGMs from a Box or a Bag Another effective way to transport HGMs is by a vacuum transport system (Figure 10.8). As opposed to diaphragm pump transfer, vacuum transfer is a pull-only conveying system that operates at a negative pressure. It may use a venturi, two-stage fan, or a positive displacement blower (illustrated) to move the air that carries the material. The vacuum system will move material at higher line velocities than a pump system. The advantage is that it does not emit particles into the work area. The vacuum system is also not prone to line plugging problems. The primary filter is usually cleaned with pulsed, high pressure, clean, dry air. A secondary

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Vacuum blower

Reverse pulsed filter

Pinch valve Secondary filter

Box

Site tube

Site panel

Air pad

Scale HV

SV

F

Discharge aeration 2 psi

Figure 10.8 Vacuum transfer system. Reconstructed from Ref. [2] with permission from 3M.

filter is placed after the receiver filter in order to protect the fan or blower. An adjustable vacuum relief valve regulates vacuum in the receiver. Typical suction is 50e100 in of water column. A sight panel and aeration of the hopper is suggested. Aeration pads are typically offset away from the center axis, near a corner of a pyramidal hopper and at different heights in the transition section. The transition section is the converging section from the hopper vertical section to the discharge valve. With hopper aeration, bubbles will flow easily. This has the advantage of reducing height for fitting into a height-limited area above a mixer. Typically, bubbles are metered continuously to a process using a loss-inweight screw feeder. Batches processed are charged by weight from a hopper on load cells or by placing the box on a floor scale. It is important that the screw feeder utilizes a variable flight depth and pitch screw as shown in Figure 10.9. Powders and other bulk solids tend to segregate when the particles are different, in size, shape, and/or density. Most segregating materials are

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Non-uniform discharge

Constant flight depth and pitch screw

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Uniform discharge

Variable flight depth

Variable pitch screw

Figure 10.9 Nonuniform and uniform discharge using screw feeder. With permission from 3M.

free flowing or slightly cohesive so that the particles can easily separate from each other. On the contrary, the behavior of poorly flowing bulk solids (materials containing fine particles or moisture) is dominated by interparticle adhesion forces, which reduce the mobility of individual particles and, thus, the tendency to segregate. Usually one wishes to avoid, or at least reduce segregation, since many downstream processes require a product with constant composition. For example, if the bulk solid must be filled into packages for customers in small quantities of equal composition, or if in a process (e.g., grinding or combustion) steady-state conditions can be attained only with a feed of constant composition. Furthermore, segregation can result in inaccurate volumetric dosing due to a fluctuating bulk density. During use of either a vacuum transfer system or pump, handling of the dusting at the receiving vessel (separation of air and HGMs) needs to be considered. The system would need to be tightly sealed with a dust collector at the receiving vessel using a dust sock or reverse jet pulse pleated filter system.

Bulk Bag Discharging Bulk bags are a common way of packaging HGMs. The most dust-free method of discharging bulk bags is to place the bag into a bag unloading station. The stations include a glove box hopper with a window and a light and flexible dust containment cover (Figure 10.10). The flexible hopper top is slightly stretched by the bag weight in order to minimize dust when the bag discharge spout is opened. The station can be enclosed on three sides. Usually, dust collection vents are placed above the top of the hopper. If desired, the station can include a weigh cell system for metering

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Figure 10.10 Balancing fluidization for pump transfer and feeding into mixing equipment [3]. Reconstructed with permission from 3M.

material to the process by weight. Capacitance sensors are used to detect material level. Often for mix tanks, the bulk bag is discharged directly into the entry way of the vessel. This requires an auxiliary filter port in order to collect the dust created during bag discharging. Material is usually transferred by vacuum suction from a vacuum receiver or a double diaphragm pneumatic pump. In the preceding sections, we emphasized the importance of fluidizing the HGMs via air assists, wands, or air pads for efficient transfer via a diaphragm pump or a vacuum transfer system. Although fluidizing HGMs is crucial for pump or vacuum transfer operations, highly fluidized HGMs may cause problems during their incorporation into mixing systems such as side feeding into a twin screw extruder. Highly fluidized HGMs, when intersected with a viscous melt stream, could be pushed back due to their increased rotational motion in their fluidized state. On the other hand, HGMs with balanced flow have enough translational motion that could overcome the force exerted by the flowing melt stream (Figure 10.11).

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H OLLOW G LASS M ICROSPHERES Highly fluidized HGMs Melt stream in an extruder

Melt stream in an extruder

Balanced flow HGMs

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Figure 10.11 Balanced versus fluidized flow of HGMs in a process.

Cohesiveness and fluidizability of HGMs may vary depending on the manufacturer of the HGM. However, for very fluidizable HGMs with long air retention time, a more balanced flow can be achieved by allowing the HGMs to settle for a period of time in day hoppers in order to increase cohesion before being side stuffed. In addition, HGMs free falling from tall silos or hoppers into a side stuffer should be avoided, since free-falling HGMs will readily be fluidized and may cause problems in feeding into melt streams such as in an extruder. Angle of repose for HGMs, that is, angle which is formed by a pile of solid with the horizontal, is about 40 e50 in their packed state but 0 in their fluidized state.

References [1] 3M Technical Paper. 3M Glass Bubbles Metering and Mixing Guide. [2] 3M Technical Paper. 3M Glass Bubbles Box Unloading Solutions. [3] 3M Technical Paper. 3M Glass Bubbles Bag Unloading Solutions.