Greg Mehos, President | Greg Mehos & Associates, LLC
Agglomeration is the process of converting fine particles into larger ones by applying external forces. Size enlargement is often desirable because agglomerates have less dust, show improved flowability, and require less volume for storage or transport.
Agglomeration technologies include pressure agglomeration, accomplished by equipment such as roller compactors, extruders, and pellet mills, and tumble-growth agglomeration, which is accomplished with equipment such as pin mixers, plough mixers, and disk pelletizers. Tumble-growth agglomeration is preferable when the final product must have sufficient strength to survive packaging and storage but must readily disperse, for instance disintegrate into primary particles, when used by the customer.
This column focuses on tumble-growth agglomeration, which is often called wet agglomeration because the process involves mixing the fine powder with a liquid. Fine powder, a liquid (usually water), and a binder (often an aqueous solution or dispersion) are fed into a wet agglomerator as separate streams. The newly agglomerated, or “green,” particles exiting the agglomerator are then fed into a dryer to remove the moisture. The optimal amount of liquid added to the powder during the agglomeration process is a fraction of the material’s full saturation. If too much liquid is added, the mixture begins to behave as a slurry. If there’s not enough moisture, the green pellets will be weak and the final, dried agglomerates will contain excessive dust. An optimum moisture content is present when the dried material has its greatest strength and its lowest dust content.
Maintaining the proper feed ratio of solids to liquid is critical to this type of agglomeration. Providing a steady feedrate of liquid into the agglomerator can be done using a pump and control valve so managing the liquid is somewhat straightforward. Ensuring a steady feedrate of fine powder, however, can be challenging.
Promoting flow to agglomeration equipment
The powder discharge rate in most agglomeration processes is typically modulated by a feeder, such as a rotary valve or screw feeder, beneath a surge or feed vessel such as a hopper, bin, or silo. If the vessel isn’t properly designed, flow problems such as arching, ratholing, or an insufficient solids discharge rate may occur. Arching occurs when the stress on the powder at the hopper outlet isn’t strong enough to cause the arch to fail. Ratholes form if the hopper walls aren’t steep enough, have too high a friction rate to allow the powder to flow along them, or both. The solids discharge rate can be limited, and therefore insufficient, if the vacuum that naturally forms near the hopper outlet causes air to flow counter to the solids flow and disrupt it. In general, a fine powder will have high cohesive strength, high wall friction, and low permeability, and hoppers with impractically large outlets require extremely steep walls to prevent flow obstructions and allow the desired throughput. The poor flowability of fine powders, after all, is often a prime motivation for agglomerating the material.
Improving powder flow to an agglomerator must begin with identifying a material’s flow properties. Andrew Jenike developed methods to measure the fundamental flow properties of bulk solids and procedures for designing hoppers, bins, and silos for reliable powder flow. His work is summarized in Jenike’s “Storage and flow of solids. Bulletin No. 123,” which is available for no cost.1
Measurement of a material’s cohesive strength and compressibility allows determination of the hopper outlet size required to prevent obstructions to flow, and by measuring wall friction (friction generated by the material moving against the vessel wall), a designer can specify the hopper angle required to prevent ratholes from forming. And by measuring material permeability and compressibility, an engineer can calculate the outlet size that allows the desired solids discharge rate.
Fluidization is another option that’s sometimes used to try and overcome flow problems associated with fine powders when agglomerating. Gas (or air) is forced through the powder bed to fluidize the material and improve flow; however, many fine powders are challenging to fluidize. Geldhart’s powder classification chart, as shown in Figure 1, indicates that fine powders with too low a particle density are too cohesive to fluidize. Injected gas will channel through the powder bed rather than fluidize it. Even when fluidization is possible, stable ratholes can develop unless the outlet of the converging section of the hopper, bin, or silo is impractically large.
An alternate method to feed fine powders into agglomeration equipment is to use a double diaphragm pump beneath a conical hopper fabricated with gas-permeable walls. While double-diaphragm pumps are common, hoppers with permeable walls are less so. Such an assembly is shown in Figure 2. Double diaphragm pumps are often used to transfer powders that 1) are dry, 2) have a particle size smaller than 150 mesh (106 microns), and 3) have a bulk density less than 800 kg/m3. These positive displacement pumps use two flexible diaphragms that create a temporary chamber that draws in and expels the powder.
A steep cone, for instance one with walls that are 30 to 45 degrees from vertical and that allows gas to be injected through its walls, reduces a fine powder’s cohesive strength at the hopper outlet, where the gas velocity is highest, and lowers wall friction by creating a layer of gas between the powder and the walls. Only a small amount of gas needs to be injected through the walls. Provided that the hopper walls are steep, gas will flow preferentially toward the outlet when the pump is running and will not disrupt solids flow.
Insight into how gas assistance and a double-diaphragm pump is effective for providing a steady solids flowrate into an agglomeration process can be gleaned by examining relationships that describe gas flow and solids flow. Because the gas is flowing through a moving bed of solids, its flowrate can be described by Darcy’s Law:
where vg and vs are the gas and solids velocities respectively. (The difference is often called the slip velocity.) C is the permeability; η is the gas viscosity; and dP/ dz is the gas pressure gradient.
The solids discharge velocity at the outlet can be described by:
where B is the hopper outlet diameter, g is acceleration due to gravity, ρbo is the bulk density of the powder θ’ is the hopper angle referenced from vertical, and the subscript o denotes the hopper outlet.
The total volumetric flowrate Q is related to gas and solids velocities by:
In addition, the double-diaphragm pump has a performance curve that relates the total volumetric flowrate to the difference between the discharge pressure (approximately zero if the stream is fed directly into the agglomerator) and the suction pressure, which depends primarily on the gas pressure drop through the hopper.
Considering equations 1 through 3 — and noting that the volumetric gas rate through the double-diaphragm pump, which can be determined from its performance curve, will be constant provided that the suction and discharge pressures remain constant — shows that the solids discharge rate from the hopper and, hence, the feed to the agglomerator will remain constant as long as the gas pressure gradient in the hopper is steady. Because a significant fraction of the total pressure drop is in the hopper’s conical section, where the gas velocity is the highest, the suction pressure can be kept nearly constant if the hopper’s powder level remains above the cone–cylinder junction. Provided the hopper walls are steep enough and the friction rate is low enough to allow mass flow (flow that delivers a first-in, last-out flow in the vessel), the stress that accompanies adding fresh powder
to the hopper isn’t transmitted to the solids at the hopper outlet, and powder can be added periodically without disrupting the process. Mass flow is likely because the gas injection method mentioned greatly reduces wall friction.
The following case study illustrates the effectiveness of using a double-diaphragm pump and a gas-assist hopper to agglomerate fumed silica. A hopper with walls 20 degrees from vertical fabricated with gas-permeable material was filled with fumed silica. A flexible line that was attached to a double-diaphragm pump was installed beneath the hopper. The outlet of the double-diaphragm pump was connected to a flexible line that fed a pin mixer. A peristaltic pump was used to feed liquid into the mixer. Figure 3 is a photograph of the setup.
After a small amount of gas was introduced to the membrane of the hopper, compressed air was fed to the double-diaphragm pump, which fed fumed silica and air, drawn through the material bed, into the pin mixer. Liquid was then pumped into the mixer at a constant rate. Bags of fumed silica were periodically emptied into the hopper to prevent the level of powder inside the hopper from reaching the cylinder–cone junction.
Samples of agglomerated silica were taken from the pin mixer outlet at regular intervals and tested for solids content. Sample results are shown in Figure 4. The standard deviation of the solids content was approximately 4 percent of the mean, illustrating the steady
discharge rate of fumed silica, which is a challenging material to handle and transfer from the hopper into the pin mixer. Improvements could be made by placing the hopper on a load cell to measure its loss in weight over time. Using a control system to vary the compressed air pressure to adjust the pump speed also could improve the process, but the relatively constant solids-to-liquid ratio achieved in this example allowed agglomerates with high strength and low dust to be consistently produced.
- Free download of Jenike’s “Storage and flow of solids. Bulletin No. 123” can be found at https://www.osti.gov/biblio/5240257.
For further reading
Greg Mehos, PE, is president of Greg Mehos & Associates, LLC, (978-799-7311, www.mehos.net). He holds a BS from the University of Colorado, a master’s degree from the University of Delaware, and a PhD from the University of Colorado, all in chemical engineering. He’s been working on powder handling projects for more than 25 years.
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