Fluidization and air-assisted discharge are two ways to optimize fine-solids handling. Choosing the proper technology is key to the stable, efficient operation of your system.
The safe and efficient operation of solids-handling equipment depends on a continuous feed of material at an accurately controlled rate. The flowability of powders and their flow behavior under pressure, temperature and humidity are key considerations in handling and processing operations, such as storage in hoppers and silos, transportation, formulation and mixing, compression, and packaging. Sometimes, small differences in moisture content, particle size, storage time, and even temperature can make a big difference in flowability; thus, it is necessary to be aware of the effects of these variables and design for the worst-case scenario.
The ideal approach to determining the flowability of a given system depends on the process requirements (i.e., the required feed consistency) and the characteristics of the material to be handled. This article offers a proven methodology for choosing the appropriate gas-injection technology for the optimization of fine-solids-handling systems.
There are several powder characteristics that qualify a powder as poor-flowing, the most obvious of which is cohesive strength, which can result in flow stoppages. A less recognized characteristic, unique to fine powders, is low permeability, which can affect the flowrate. The rate at which a fine powder will flow through an opening can be increased by various methods of gas injection, ranging from very small quantities to large volumes. For instance, a gas-permeation system can increase solids flowrates to double or triple the gravity-only rates, using very small quantities of gas.
In a different technique, known as air-assisted discharge, injected gas entrains the solids, and the solids-gas mixture is purged from the vessel at rates of up to an order or magnitude higher than possible without any gas, while still maintaining a consistent solids density.
Complete fluidization of a vessel is another method by which discharge flowrates of difficult-to-handle powders may be increased even more dramatically - two or three times the unaerated value. However, not all powders are suitable for fluidization. Some are highly sensitive to the amount and method of gas usage, and may exhibit erratic flow behavior. Furthermore, fluidization may raise capital and operating costs, due to [the purchase of] additional gas-handling and ancillary equipment, increased gas usage, the lower bulk density of discharged material, and the potential for particle segregation.
Sizing outlets
The minimum outlet size for a vessel that stores bulk solids is primarily determined by the requirements to prevent arching (also called bridging) and to achieve the discharge rates needed to satisfy downstream demand. A bulk solid will form an arch over the outlet of a vessel if the cohesive strength of the bonds between particles exceeds the stresses acting on that arch (1). The level of stress is directly proportional to the outlet size; hence, once the cohesive strength of a material has been measured, the minimum outlet size required to prevent an arch from forming can be calculated.
In a majority of cases, the minimum outlet size is also large enough to provide the required discharge rates. But, when handling fine powders, it is sometimes challenging to meet this demand. In such instances, gas injection into the vessel, if done correctly, is an effective way to increase powder flowrates.
Flowrates for coarse materials
Sometimes, small differences in particle size, moisture content, composition, and in storage conditions, such as relative humidity, temperature, And storage time can make a big difference in flowability. Thus, it is necessary to be aware of the effect of these variables and design for the worst-case scenario.
Let us assume that a vessel with a conical hopper (at a 20-deg. angle from vertical) and a 12-in.-dia. outlet is used to store plastic pellets (γ = 32 lb/ft^sup 3^). Equation 1 predicts that the maximum discharge rate from this vessel would be 118 lb/s (or 213 ton/h).
Although this relationship works quite well for most coarse materials, it does not consider other important material attributes, such as cohesive strength, particle size, density and shape. In light of the fact that Eq. 1 only accounts for body forces due to gravity, it cannot be used when other forces act on the particles.
Under certain conditions, the particles can also be subjected to body forces due to gas-pressure gradients within the bulk solid. If the particles are large enough to create sufficient void space so that interstitial gas can move freely, the movement of gas does not impose a significant force on the particles. However, this is not the case for fine powders. As a powder moves through a vessel, its bulk density - and, hence, its void ratio - changes, thereby creating pressure gradients, which, near the vessel outlet, form in a countercurrent direction to material flow (3). Thus, forces are exerted on the solids in a direction opposite to that of gravity, which can significantly reduce the rate of discharge from a vessel.
While there is no clear distinction between what is considered a "coarse" material and what is considered "fine," it has been shown that the effect of gas movement becomes a dominant factor in powder behavior when powders contain a large percentage of particles measuring less than 100 μm.
If the same vessel described earlier were used to store resin powder (mean particle size = 300 μm γ = 32 lb/ft^sup 3^), Eq. 1 would again predict a discharge rate of 213 ton/h. In reality, this rate is much higher than what can be achieved with resin powder in a mass flow vessel because of the two-phase (gas/solids) effects.
Permeability and compressibility are the two properties of a bulk solid that determine the extent to which two-phase flow effects influence a material's behavior. While both of these properties are strong functions of particle size, there is no direct way to calculate them from particle-size information unless the particles are uniformly sized, incompressible, and regularly shaped (e.g., spherical). However, one can measure permeability and compressibility directly.
Equation 2 is valid in the laminar flow region, where fine powders are subjected to the significant effects of interstitial gas-pressure gradients. For high-velocity gas flows through coarser beds (e.g., plastic pellets), a non-linear turbulent model should be used.
Once the permeability and compressibility of the material are included in flowability assessments, it is no longer possible to use a closed-form solution, such as Eq. 1, to calculate maximum flowrates. Instead, a numerical solution must be applied. The authors have developed a model that predicts an average discharge rate of 15 ton/h of resin powder in the example above; in stark contrast, Eq. 1 predicts a discharge rate of 213 ton/h.
Gas permeation systems
The discharge rate of fine powder from a vessel can be increased by permeating small amounts of gas into the hopper to alter the gas pressure gradients (3). Continuing with the resin powder example, the injection of 1 ft^sup 3^/min of air into the hopper will increase the discharge rate to 28 ton/h - an 87% increase. However, if too much air is injected, the flow of solids becomes unsteady, and the risk of uncontrollable discharge becomes very possible. In this scenario, a gas permeation rate of 2 ft^sup 3^/min would cause extremely unstable solids flow. In general, the use of a gas permeation system can increase steady-state "gravity-only" flowrates of fine solids by a factor of two or three. Unfortunately, there are many industrial applications where the required discharge rates are significantly higher than what can be achieved even with a gas permeation system.
Another way to increase the discharge rate of fine solids is to increase the size of the vessel outlet; however, this option may become cost-prohibitive. In such situations, fluidization or air-assisted discharge may be the better options to significantly increase the discharge rate of fine powders.
Fluidization behavior
A large body of research has been developed around fluidization in the petrochemical and chemical processing industries (5, 6). Fluidization occurs when upward flow of a fluid (gas or liquid) through a bed of bulk solid particles reduces particle-to-particle friction and causes the bulk material to behave as a fluid.
The fluidization characteristics of a solid are typically presented in a graph that plots the pressure gradient over the bed as a function of superficial fluid velocity. Figure 1 illustrates the behavior of an ideally fluidizable bulk material. Two variables are plotted: pressure gradient (circles) and bulk density (triangles) of the powder. In this example, gas is fed at increasing rates across a 12-in.-high bed of powder, until the flowrate reaches a maximum. Then, the flow is decreased back to 0 ft/s. With the increasing gas flow, the pressure gradient across the bed first increases linearly, then reaches a peak, and subsequently drops. For materials that fluidize well, the pressure gradient levels off after it drops, and remains fairly constant with increasing gas flow. When the pressure has remained steady for a number of incrementally increasing gas flowrates, the gas velocity is decreased in a stepwise fashion. The other measurement shown in Figure 1 is bulk density, which decreases as the column height increases due to fluidization.
Leva (7) defined minimum fluidization velocity (U^sub mf^) as the point where lines A and B intersect. To the right of this point, material is fluidized. Further increases in gas velocity cause more-vigorous fluidization, until finally the particles are conveyed upward, signifying that complete fluidization (U^sub cf^) has been achieved. Most bulk materials only approximate this behavior. A more typical pattern is shown in Figure 2.
The Geldart chart (Figure 3) has been used successfully to categorize the fluidization behavior of materials. Introduced by Geldart in 1973 (8), it divides bulk materials into four categories based on mean particle size and particle density (table). Group A materials are considered aeratable and demonstrate fluidization behavior closest to ideal. Group C materials are finer and tend to be more cohesive and difficult to fluidize. Group B and Group D materials are coarser and require more gas to fluidize, and, they are more likely to exhibit unstable behavior, such as slugging.
A drawback of the Geldart chart is that it does not account for many of the particle characteristics that influence powder behavior, including size distribution, shape, elasticity, cohesiveness and composition, or the storage conditions, including relative humidity, temperature and storage time. In particular, small differences in moisture content, particle size and storage time can make a big difference in flowability.
In material-handling applications, fluidization is often used to overcome flow problems with materials that fall into Groups A, B and C. Group D materials and some larger-particle-size Group B materials are too permeable for fluidization to improve flowability. However, the Geldart chart does not provide quantitative information about the volume of gas that may be required to fluidize a material, or how fluidization will affect solids-handling.
While much work has been done to develop models to calculate the fluidization properties of materials, predicting their actual behavior is not an exact science and requires experimental corroboration. The setup in Figure 4 is used to determine if a material is suitable for discharge from a fluidized bin. Gas flowrate and pressure into the fluidizer are controlled, and solids discharge through the rotary valve is measured for various gas pressure and injection rates. The behavior of the solid is typically observed to determine the uniformity of flow, the extent of (if any) gas channeling, the vessel's ability to empty all or most of the contents with gas flow alone, its ability to re-fluidize after the contents have dearated and remained at rest for some time, etc. The data provide useful ranges of pressure, gas injection and solids discharge flowrates, and the outlet sizes needed to achieve the desired discharge rates, all of which may be used for the design and scaleup of material-handling systems.
Fully fluidized systems
Fluidization has been applied widely in the chemical industry for process vessels and reactors where the advantages of intimate gas contact, heat transfer, and mixing are desirable. These processes almost always operate at conditions well above the minimum fluidization velocity and, in the case of circulating fluidized beds, at velocities high enough to transport the solids. However, when fluidization is used to improve powder flowability, it is desirable to use the lowest possible velocity to achieve the desired result, so as to minimize energy input. This often results in fluidization conditions slightly above the minimum fluidization velocity or conditions where most of the solids are not actually fluidized.
The conditions that work best for a given application depend on the properties of the material and the requirements of the solids-handling system. One type of system, shown in Figure 4, relies on maintaining the entire contents of the vessel in a fluidized state. Discharge is controlled by a rotary valve or other device that can seal against fluidized material. Pressure and gas flowrate are usually selected as the minimum values necessary for reliable operation. Gas flow in this system is predominantly upward. The feeding device acts as a seal, so there is relatively little gas movement (and hence little driving force) radially from the periphery of the vessel toward the outlet. If the solids are sufficiently fluidized, they will flow reliably through the outlet without plugging or forming a rathole (an empty vertical channel above the outlet). This type of system is used successfully for Geldart Groups A, B and C materials, although many Group B materials may not require fluidization for reliable handling. Furthermore, some Group C materials are difficult to fluidize without mechanical agitation.
Reliable operation of a fluidized system depends on maintaining the material in a fluidized state. If gas flow is reduced and the material defluidizes, flow can become erratic and may stop completely. The preferred configuration for a fully fluidized system is a cylinder mated directly to the fluidizing membrane, because it provides constant gas flow through the bed of solids. However, this limits the system's size and capacity. If additional storage is required, it is possible to use a configuration where the fluidizing membrane is placed below a conical section on top of which a larger-diameter cylinder may be placed.
Air-assisted dischargers
Many powders can be handled in systems where only a small amount of material in the system is fluidized. These systems, called air-assisted dischargers, are distinctly different from fluidized systems described above. The primary distinguishing feature of this system is that the gas velocity in the bed of material above the membrane is below minimum fluidization conditions.
Air-assisted discharge systems generally operate best with a discharge valve that opens fully to provide unrestricted flow (i.e., no sealing feeder such as a rotary valve). This allows substantial gas flow radially from the periphery of the membrane toward the outlet. This radial pressure gradient moves material toward the outlet and causes a uniform flow pattern above the membrane.
When discharge is restricted by a rotary valve or other metering feeder, solids flow can be significantly faster in the center, with little or no movement at the periphery. The image in Figure 5 shows the flow pattern that developed in the system depicted in Figure 4 when flour (Geldart Group C) was discharged through the membrane at a velocity significantly less that minimum fluidization velocity. The flow channel in the center resembles a typical funnel-flow pattern from a gravity-flow bin. Even though air is introduced through the entire membrane area, flow occurs only directly above the outlet.
While this type of air-assisted discharge may provide significantly higher discharge rates and overcome rate-limitation problems that would occur in non-aerated gravity flow, it may still experience many of the flow problems associated with funnel flow, such as increased effects of particle segregation, dead regions of material, and the potential to form ratholes. To avoid these flow problems, a fully open, unrestricted on/off valve must be used, or the system must be operated as a fully fluidized system with higher gas flowrates.
Air-assisted discharge systems have much lower gas flow requirements than comparable fully fluidized systems. The gas supply is set so that the pressure below the membrane is insufficient to provide enough flow to fully fluidize the material. However, when the solids discharge valve opens, gas flow increases substantially because it can freely flow from the membrane to the unrestricted discharge opening. This relatively high rate of flow fluidizes material adjacent to the membrane and provides a driving pressure gradient to push material toward the outlet. This flow occurs only when the discharge valve is open; when the valve is closed, gas pressure below the membrane is insufficient to fluidize the material.
Figure 6 shows the discharge pattern for flour exiting from an air-assisted system via an on/off discharge valve. The system is identical to that used for the test shown in Figure 5, with the exception of replacing the rotary valve with a ball valve. The air pressure applied to the membrane was the same as that used for the setup in Figure 5, but discharge occurred through the fully open valve. The discharge pattern is more uniform, and it is clear that the colored material moved from the edges toward the central discharge point. There is also no evidence of bubbling or gas channeling, as in Figure 5.
Gas pressure settings, outlet size, and the maximum solids discharge rate for this system are determined from tests. Air-assisted dischargers can be used on large silos to achieve mass flow at high discharge rates that would be difficult if not impossible to achieve with conventional, non-aerated feeders. To ensure reliable flow, design of the bin and hopper above the aerated section must be based on the flow properties of the material. Simply adding a large air-assisted discharger to a poorly flowing vessel will not necessarily correct all flow problems. Many materials that respond well to aeration have significant cohesive strength when deaerated and may require a mass flow vessel to provide reliable flow.
Air-assisted discharge has been used widely for handling cement, fly ash, alumina and other similar materials in large storage silos. Using an array of permeable membrane air distributors at the bottom, maximum storage volume is achieved in an efficient structure with minimum height. These storage structures can be very large (over 100 ft in diameter) and in most cases, provide reliable discharge of stored solids with little stagnation. Recovery from this type of system usually occurs in stages by activating a section of the membranes and recovering material from one or more outlets in sequence.
One common misconception about the operation of this type of system is that flow occurs along the entire length of an active membrane. In general, flow occurs only in a relatively small zone that lies directly above an active outlet, even though a large area of the silo floor may be receiving air. This occurs for the same reason that the flow channel in Figure 5 develops in a narrow region above the outlet.
When designing a fully fluidized or air-assisted discharge system, the following must also be considered:
* the membrane must have sufficiently low permeability to effectively distribute air and thereby minimize channeling
* depending on the size of the fluidizer or air-assisted dis charger, partitioning and valving may be necessary to ensure reasonable air distribution
* the housekeeping load may increase because any type of air addition will result in a greater potential for dust to escape from the system
* air used for handling must be vented from the top of the bin, as well as from downstream equipment; adding air to a system that previously used gravity flow may require additional filtration or venting
* air must be dry if the material is hygroscopic
* it is necessary to use inert gas if hazards exist with air
* segregation may be a concern
* the downstream equipment must be able to handle aerated material.
RELATED COURSES OFFERED BY AlCHE/ASME IN 2006 INSTRUCTED BY JENIKE & JOHANSON
Course #CH032: Flow of Solids in Bins, Hoppers, Chutes, and Feeders, Level 1; Sept. 20-21,Las Vegas, NV; Dec. 5-6, Atlanta, GA
Course #CH033: Pneumatic Conveying of Bulk Solids, Level 2; Sept. 22, Las Vegas; Dec. 7, Atlanta
For more information, please contact David Tonn at: (212) 591-7303 or tonnd@asme.org.
[Reference]
Literature Cited
1. Jenike, A. W., "Storage and Flow of Solids." Univ. of Utah Engineering Experiment Station, Bulletin No. 123 (Nov. 1964).
2. Johanson, J. R., "Method of Calculating Rate of Discharge from Hoppers and Bins," Transactions of Society of Mining Engineers, 232 (Mar. 1965).
3. Royal, T. A., and J. W. Carson, "How to Avoid Flooding in Powder Handling Systems" Powder Hundling and Processing, Trans Tech Publications, 5 (1), pp. 63-67 (Mar. 1993).
4. Carson, J. W., and J. A. Marinelli, "Characterize Bulk Solids To Ensure Smooth Flow," Chem. Eng., 4 (101), pp. 78-90 (Apr. 1994).
5. Zenz, F. A., and D. F. Othmer, "Fluidization and Fluid Particle Systems." Reinhold Publishing Corp., New York, NY (1960).
6. Kunii, D., and O. Leveaspiel, "Fluidization Engineering," John Wiley and Sons, Hoboken, NJ (1969).
7. Leva, M., "Fluidization," McGraw Hill, New York (1959).
8. Geldart, D., "Types of Gas Fluidization," Powder Technology, 7, pp. 285-292(1973).
[Author Affiliation]
HERMAN PURUTYAN
JOHN W. CARSON
THOMAS G. TROXEL
JENIKE & JOHANSON, INC.
[Author Affiliation]
HERMAN PURUTYAN is vice president of Jenike & Johanson, Inc. (400 Business Park Drive, Tyngsborough, MA 01879; Phone: (978) 392-0300; Fax: (978) 392-9980; E-mail: hpurutyan@jenike.com). Since joining the firm in 1991, he has designed reliable handling systems for a wide range of materials for the food, pharmaceutical and chemical industries. Purutyan earned BS and MS degrees in mechanical engineering from Worcester Polytechnic Institute (Worcester, MA), and an MBA from Babson College (Wellesley, MA) and is the holder of two patents. He is a member of the ASME Structures for Bulk Solids Committee.
JOHN W. CARSON is president of Jenike & Johanson (E-mail: jwcarson@ jenike.com), where he has been active in research, consulting and management of the company. Carson received a BS in mechanical engineering from Northwestern Univ. and a PhD in mechanical engineering from Massachusetts Institute of Technology. He is a member of AlChE, ASME, ASCE, and ASTM International, and is a founding member of AlChE's Powder Technology Forum.
THOMAS G. TROXEL is vice president of Jenike & Johanson (E-mail: tgtroxel@slo.jenike.com). Troxel has been involved in many aspects of the firm's consulting and research activities on a wide range of projects, including flow-properties testing, modeling, blending, pneumatic conveying and fluidization. He has been a major force behind the firm's expansion of service in the areas of mechanical design engineering and supply of custom-built equipment. Troxel has a BS in engineering from California Polytechnic State Univ. (San Luis Obispo, CA).
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