Powder and Bulk Engineering, November 2000 71

Pneumatic points to ponder...

Building ontheinformationinhis first four series of column~,l.~ Paul E. Solt, a private consultant with more than 43 years experience in- stalling and troubleshooting pneumatic conveying systems, presents a fifth series, on the gen- eral application of pneumatic conveying. In a separate section, Solt also answers your pneumatic conveying questions.

n this eighth column on the gen- eral application of pneumatic con- I veying, we’ll apply information

from previous columns to discussing equipment for handling fluidizable material. This material falls into one of the three general categories of dry bulk materials, the other two being free-flowing (granular) and nonfree- flowing (cohesive, stringy, and lumpy) materials. Fluidizable mate- rial has unique handling characteris- tics that not only provide some processing advantages but allow the material to be pneumatically con- veyed in equipment other than pneu- matic pipelines.

While fluidizable material has been mentioned in many previous columns,

certain columns have provided more in-depth information about it. The March 1995 and November 1998 columns define fluidization as part of a general discussion of material charac- teristics, and the March 1994 column describes a feeder that handles flu- idized material. As you read this month’s column, it may be helpful to review information in these columns.’.*

Fluidization basics Fluidization occurs when a small amount of air (or other gas) is passed upward through a fine bulk material of mixed particle size, which suspends (or floats) each particle on an air cush- ion and reduces the interparticle fric- tion. This gives the air-solids mixture a liquid characteristic - hence the termfluidization. To be fluidizable, the material must have a wide particle size range, and the mean particle size must be substantially smaller than the largest particle size. For example, a typical fluidizable material may have a 90-micron (0.0035-inch or 170- mesh) mean particle size and a 250- micron (0.0098-inch or 60-mesh) maximum particle size.

If you pass a small volume of air through a bed of fluidizable material, the individual particles will be sus- pended in the air, reducing the inter- particle friction and making the air-solids mixture act like a liquid. This fluidized mixture will have all the characteristics of a fluid, includ-

ing hydrostatic pressure, viscosity, and flowability on a shallow incline.

Air retention time. Some fluidizable materials can remain fluidized for a time after the airflow through the bed has stopped, deaerating slowly. Such a material has a long air retention time. Examples include fly ash, ce- ment, some clays, pulverized lime- stone, and pulverized coal. Other fluidizable materials have a very short air retention time and quickly lose their fluidization. An example is alu- mina. Although alumina is one of the easiest materials to fluidize, it deaer- ates and settles very quickly, becom- ing a solid mass in just seconds.

Low-velocity airflows, tranquil flu- idized beds. This column concen- trates on fine materials that fluidize with low-velocity airflows passing upward through the material, forming a tranquil fluidized bed. The airflow velocity can be as low as 2 fpm and is barely perceptible as the air exits from the material bed’s top. Higher airflow velocities and air pressures can flu- idize much coarser materials, but in- stead of forming a tranquil fluidized bed, the higher velocities and pres- sures form a violently boiling bed in which fines are blown out and small geysers of material form.

While previous columns have dis- cussed conveying fluidizable mate- rial in a pneumatic pipeline, the next

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72 Powder and Bulk Engineering, November 2000

section will explain how you can pneumatically convey this material in a trough called an air gravity con- veyol: The final section explains how fluidized vessels can apply fluidiza- tion to this material to enhance con- ventional pneumatic conveying in pipelines or condition the material for subsequent handling or processing. These vessels can promote the mate- rial’s discharge from a bin into a pneumatic conveying system or process, can segregate, blend, heat, or cool the material, or can control the material’s flow to a conveying system or process.

Air gravity conveyors A fluidized material’s liquid-like char- acteristics allow it to flow down an in- cline, just as rainwater will flow down a downspout. An air gravity conveyor makes use of this characteristic to move the fluidized material from one point to another. One of the con- veyor’s major advantages over a pneumatic conveying system is that it moves the material at a relatively low velocity. This greatly reduces or elimi- nates conveying system wear, making the air gravity conveyor especially suitable for an abrasive fluidizable material. The conveyor offers still other advantages over a pneumatic conveying system: It uses a very low- pressure air supply and low horse- power, requires little maintenance because it has no moving parts, and is less expensive.

A fluidized material’s liquid-like characteristics allow it to flow down an incline, just as rainwater will flow down a downspout.

The air gravity conveyor, shown in Figure la, has an enclosed, down- ward-sloping trough with a plenum chamber beneath an air-permeable fluidizing member that runs the con- veyor’s entire length. The conveyor includes one (or more) inlet and outlet and can include turns (as shown in Figure lb), flow diverters (to direct

flow to other conveyor legs), flow- control gates (at or near the inlets), and cut-off gates (typically at the in- lets or in each leg after a flow diverter to cut off flow).

In operation, material is fed into a flu- idizing hopper above the conveyor’s inlet and from there discharges into the conveyor and flows downward along the fluidizing member. Air is supplied into the plenum chamber and passes upward through pores in the fluidizing member, maintaining the material’s fluidization as it flows to the conveyor’s outlet. There’s no limit to the conveyor’s length as long as enough headroom is available to allow the conveyor to be inclined and air is supplied along the conveyor’s entire length.

Selecting the conveyor for your flu- idizable material. To design such a conveyor for your fluidizable mate- rial, you need to know the material’s poured andfluidized angles of repose. You can measure the poured angle of repose by pouring a material sample onto a level surface so that it forms a cone and then measuring the angle to the horizontal formed by the cone’s surface.

Your material’s angle of repose when it’s mixed with air depends on the material’s particle size, density, and shape. As the airflow velocity through the material bed increases, the mate- rial’s poured angle of repose changes and the cone of material flattens out and spreads on the fluidizing surface. With increasing airflow velocity, the

74 Powder and Bulk Engineering, November 2000

material’s angle of repose will reduce to some practical limit; at this point, increasing the airflow will have little effect on the angle of repose. This limit is called thefluidized angle of re- pose and is defined for a given rate of fluidization (airflow velocity).

Let’s consider the example of one flu- idizable material. This material has a 60-degree poured angle of repose. When air passes upward through the material pile at a 0.5-fpm velocity, the fluidized angle of repose reduces to 30 degrees. Increasing the airflow ve- locity to l fpm reduces the angle to 10 degrees, and an increase to 1.5 fpm re- duces the angle to 7 degrees. Beyond this point, any airflow velocity in- crease will create turbulence, but this material’s fluidized angle of repose won’t go below 6 or 7 degrees.

To ensure that an air gravity conveyor can handle such a material, you need to install it at an incline greater than the material’s fluidized angle of re- pose - for this example, at about 8 degrees or greater. In addition, make sure your plant has enough headroom to allow the conveyor to be fed by your process and still discharge into your receiving vessel. The air supply also must be sufficient to create the desired upward airflow through the air-permeable fluidizing member along the conveyor’s entire length. You need to make still other conveyor design decisions based on your appli- cation requirements and operation’s layout, including the placement of the conveyor’s inlets and outlets and any turns, flow diverters, flow-control gates, or cut-off gates.

Conveying a highly abrasive mate- rial. You can take advantage of the air gravity conveyor’s relatively low ve- locity compared with that of a pneu- matic conveying system to convey a highly abrasive material such as Port- land cement, fly ash, or alumina. The material can be gently elevated in a

bucket elevator or similar equipment to the conveyor’s inlet hopper. The material can then flow downward through the air gravity conveyor to ground level, where it’s elevated again to the inlet hopper of a second air gravity conveyor. This process can be repeated several times. The mate- rial’s fluidization in the air gravity conveyor reduces or eliminates con- veyor wear.

Fluidized vessels Without a top cover, the air gravity conveyor’s air-permeable membrane and plenum chamber form a typical fluidizing member. Such a fluidizing member can be used in many vessel types - often as part of a pneumatic conveying system - to fluidize material and promote its discharge to a pneumatic conveying system or process; to segregate, blend, heat, or cool the material discharged from a pneumatic conveying system; or to control the material’s flow to a pneu- matic conveying system or process.

To promote material discharge. A vessel commonly used to promote dis- charge is a railcar that has a set of air gravity conveyors on the railcar floor. The conveyors fluidize the material in the car and cause it to flow downward to one of two discharge valves at the railcar’s bottom. The fluidized mate- rial can be transported from the dis- charge valve to a storage vessel by a pneumatic unloading system.

The material’s fluidization in the air gravity conveyor reduces or eliminates conveyor wear.

Another common fluidized vessel is a bin whose cone section is equipped with a fluidizing member. The fluidiz- ing member in the cone section flu- idizes the material to eliminate bridging over the outlet or to increase

the discharge rate from the bin. The dis- charged material can feed into a pneu- matic conveying system or process.

Such a fluidized vessel can achieve good gravity flow and complete dis- charge of a material with a cone sec- tion that’s shallower than the typical 60-degree cone section. One example is a storage silo for Portland cement. When equipped with a fluidizing member, such a silo can have an al- most flat bottom - say, with a 15-de- gree cone section - and is able to almost completely discharge the ce- ment. The shallower cone section re- duces the total bin height and thus reduces its construction cost.

To segregate, blend, heat, or cool material. A vessel fitted with a flu- idizing member can also enhance pneumatic conveying by processing or conditioning the material after it exits the pneumatic conveying sys- tem, before it’s stored or discharged to a process. The fluidized vessel can fluidize the material to segregate, blend, heat, or cool it.

While the entire contents of a flat-bot- tom silo 15 feet in diameter and 30 feet tall can be fluidized by passing only a small volume of air up through the material, designing a fluidized vessel still requires understanding and re- specting some basic laws of physics.

One important law involves structural loading. When the material in a flat- bottom or cone-bottom bin is flu- idized, it changes the structural loading on the vessel. This structural loading - called hydrostatic loading - is the same as that resulting from loading water into the vessel. In many cases, converting a conventional dry storage bin into a fluidized vessel has resulted in structural damage to the vessel. To avoid this problem, make sure you design your fluidized vessel for hydrostatic loading.

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Another law for designing a fluidized vessel involves the fluidizing air sup- ply. To fluidize the vessel’s entire contents, the air you supply to the flu- idizing member must have a higher pressure than the material’s hydrosta- tic headpressure. For instance, if the material in the vessel has a fluidized density of 60 Ib/ft3 and the vessel is 30 feet tall, the material’s hydrostatic head pressure will be 60 x 30 = 1,800 Ib/ft2 (1,800/144 = 12.5 lb/in2). You must supply fluidizing air at or above 1,800 Ib/ft2 to totally fluidize the ves- sel’s contents, because air always flows from an area of higher pressure to a lower-pressure area and won’t flow the opposite way.

Controlling the fluidizing air volume will control what happens in your flu- idized vessel. For instance, supplying a minimal air volume to a vessel with a large gradation of particle sizes will

cause the coarser material to accumu- late on the vessel bottom and the finer materials to float in the fluidizing air. In this way, you can segregate the material in the fluidized vessel.

By adding fluidizing air at minimal volume over just part of the vessel’s bottom - say, only 80 percent - and adding a higher air volume over the re- maining area, you can create a flu- idized vessel that blends the material. The density of the material above the area with higher air volume will be lower because of the material’s greater fluidization and expansion in this area. But above the low-air-volume area, the material will have higher density. This difference in pressure near the vessel bottom will cause the high-pressure material above the low-air-volume area to flow into the low-pressure, high-air-volume area; this material in turn becomes fluidized and flows up-

ward. By this method, you can circu- late and completely blend an entire vessel’s contents.

By increasing the fluidizing air volume - this time, across the vessel’s entire bottom - above the minimal level to a level that will cause the largest parti- cles to float, you can create an entirely fluidized vessel. In many cases, you can heat or cool the material by adding heating or cooling coils or plates to the vessel. To withstand the substantial hy- drostatic pressure created by the mate- rial’s complete fluidization, you must ensure not only that the vessel has ade- quate structural strength but that the coils or plates are rigidly installed.

To control materialflow to a convey- ing system orprocess. You can also use a fluidized vessel to control mate- rial flow to a pneumatic conveying system or process. This is because the

fluidized material will flow through the vessel’s discharge opening just as water would flow through it. As long as you know the pressure inside the vessel and the discharge opening’s di- ameter, you can use fluid flow equa- tions to calculate the material flowrate from the fluidized vessel to the conveying system or process. [Editor’s note: You can find fluid flow equations in physics handbooks and other engineering resources, in- cluding Perry S Chemical Engineers ’ Handbook (seventh edition, ed. Don W. Green, McGraw-Hill, 1997).]

PBE

Endnotes 1. Columns in Paul E. Solt’s fist “Pneumatic

points to ponder ...” series (analyzing relationships among pneumatic conveying system operating parameters) appeared in

Powder and Bulk Engineering, November 2000 77

the March, July, and November 1989 and 1990 issues of Powder and Bulk Engineering. Second series (how to apply pneumatic conveying basics to designing a conveying system): March, July, and November 1991, March, July, and December 1992, and March 1993. Third series (design criteria for a pneumatic conveying system): July and November 1993 and March, July, and November 1994 and 1995. Fourth series (troubleshooting pneumatic conveying systems): March, July, and November 1996 and 1997 and March 1998 issues. Previous columns in the fifth series (the general application of pneumatic conveying) appeared in the July and November 1998, March, July, and November 1999, and March and July 2000 issues. See endnote 2 for information on ordering reprints.

2. Three volumes of “Pneumatic points to ponder ...” reprints are available from Powder and Bulk Engineering: Volume 1, 1989 to 1993, Volume 2, 1994 to 1996, and Volume 3, 1997 to 1999. For more infor- mation, contact Mary Watt at 612/866-2242, fax 612/866-1939 (mwatt@cscpub. corn).

Paul E. Solt is an in- dependent consul- tant specializing in pneumatic convey- ing topics for both the American Insti- tute of Chemical En- gineers (AIChE),

New York, and the Center for Profes- sional Advancement, East Brunswick, N.J. Solt has a BS in mechanical engi- neering from Lehigh University, Bethlehem, Pa., and holds several patents for pneumatic conveying de- vices. Ifyou have questions about this column or your conveying system, contact the author at Pneumatic Con- veying Consultants, 529 South Berks Street, Allentown, PA 181 04; 61 0- 437-3220, fax 610-437-7935 (pcc solt@ entemet).