Powder and Bulk Engineering, April 2001 43

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e your drying problem: ~~

r selection tips

Patrick McCarty Krauss-Maffei Process Technology

With more than 60 dryer manufacturers in the US, you face a confusing array of options when selecting a dryer. The information in this article can help you cut through the confusion to find the right dryer for your application. After discussing some dryer ba- sics, the article discusses the operation and applica- tion of common dryers as well as the factors you should consider when choosing a dryer.

nderstanding how a dryer operates starts with un- derstanding how the dryer is classified by operat- U ing mode and feed-handling capability.

Convection versus conduction Dryers can be classified by operating mode, either convec- tion (direct heat) or conduction (indirect heat), as shown in Table I. In a convection dryel; hot gas (typically air) pro- vides sensible heat to enhance the drying process. (Sensi- ble heat is that absorbed by a material during a temperature change without changing the material’s state.) One form of convection drying is particle drying, in which the hot air moves in and around the individual particles. This form of convection drying is provided by spray, flash, rotary, fluid- bed, and belt conveyor dryers. The other form of convec- tion drying is slab or beddrying, in which the material lays in a bed and the hot air flows over the bed’s surface. In this case, the material bed is so thick that diffusion of the mate- rial’s moisture to the bed’s surface controls the evapora- tion rate, and the moisture’s mass transfer is very low because minimal (or no) agitation is applied to the material

bed. Tray and rotary tray dryers provide this form of con- vection drying.

In a conduction dryel; radiant heat or contact with hot sur- faces provides the heat of vaporization to dry the material. The primary driving force for the heating process is con- duction; the vapor removal is enhanced by vacuum opera- tion or purge gas. In a dryer that can operate under vacuum, the reduced pressure in the dryer lowers the boiling point of the material’s moisture and facilitates the drying process; the vapors created in the vacuum environment are removed by a condenser-vacuum system. Examples are tray, coni- cal, rotary, and plate dryers. In a conduction dryer operat- ing with a purge gas, the vapor is removed via an air (or gas) stream directed over the material’s surface. The purge dryer operates at atmospheric pressure or slight overpressure. Examples are rotary steam-tube, drum, and plate dryers.

Feed-handling capability Dryers can also be classified by the feed material they can handle, as shown in Table 11. The feed’s consistency can be a liquid in slurry, solution, gelatinous, or pulp form; a pumpable semisolid in viscous, slimy, gelatinous, thixotropic (a gelatinous form that becomes fluid when disturbed), or muddy form; a nonpumpable semisolid in lumpy or kneadable form; orfree--owing in a fine, coarse, powder, granular, flake, fiber, crystalline, crumbly, or pul- verized form.

The feed’s consistency and heat sensitivity together deter- mine which dryer is suitable for handling it. For instance, a spray dryer can handle a heat-sensitive liquid slurry such as milk concentrate because of the dryer’s relatively short residence time.

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44 Powder and Bulk Engineering, April 2001

Operation and application of common dryer types The following information explains how commonly used dryers work and can be applied. The convection units dis- cussed here are the spray dryer, flash dryer, rotary dryer, fluid-bed dryer, belt conveyor dryer, tray dryer, and rotary tray dryer. The conduction dryers described include the conical dryer, drum dryer, and plate dryer. In some cases, both convection and conduction versions of the same dryer are available.

Spray dryer. A spray dryer is a continuous convection dryer that can handle a liquid solution or slurry or pumpable semisolid feed; this versatility makes the dryer unusual. The dryer can handle even more exotic applica- tions, including heat-sensitive feeds that can’t be mechani- cally dewatered or exposed to high temperatures for a long period and feed solutions that contain very fine particles that will agglomerate and fuse if dried in any other form than a dilute solution. Common applications include soaps, detergents, coffee extracts, pigments, and milk powders.

In operation, the feed is pumped through an atomizer at the dryer’s top; the atomizer turns the feed into droplets that flow downward inside the dryer and are dried in contact with a hot airstream. The atomizer type determines the droplet size, and, in turn, the final particle size. The hot airstream and droplets typically flow in the same direction (called cocurrentflow), which allows low-temperature drying of the particles, but can also flow in opposite direc- tions (called countercurrentflow) for higher-temperature drying. The feed’s high water content - typically 60 to 95 percent - makes this a very energy-intensive dryer that uses high volumes of hot air. The residence time is typi- cally 3 to 30 seconds and plays a critical role in achieving the final product’s quality. The dryer can dry material at rates up to several thousand pounds per hour and tends to be very large - from 30 to 50 feet tall -to handle the high evaporative load.

[Editor’s note: Find more information on this dryer in “How to optimize your spray dryer’s performance” on page 3 1 in this issue.]

Flash dryer. A flash dryer, also called apneumatic con- veyor dryec is a heated pneumatic conveying system, as shown in Figure la. This continuous convection dryer is available in single- and multiple-stage and ring units. The feed is typically a nonpumpable semisolid or free-flowing material. Typical applications are polymers, bulk chemi- cals, and starches, which require high throughput rates and very short residence times.

In operation, a feed system dispenses the feed to the dryer, where it’s entrained by a hot airstream that both dries the feed and conveys it to the discharge. The single-stage ver- sion removes small amounts of surface moisture, while the

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multiple-stage dryer can handle more difficult applica- tions such as removing large amounts of moisture from a heat-sensitive material and drying a material that contains both surface and internal moisture. In a ring dryer, a ring- shaped duct classifies coarse, moist particles and fine, dry particles and recycles the coarse to the dryer for further drying, thus better controlling the residence time.

Powder and Bulk Engineering, April 2001

With each version, the dryer’s feed system is critical be- cause the feed must be in particle form to be dispersed in the hot airstream. The feed typically has a low percentage of surface moisture, so the typical continuous flash dryer can provide very fast drying, from 50 to several thousand pounds per hour, with residence times from 0.5 to 10 sec- onds. The dryer is energy-intensive, uses high volumes of hot air, and can require a multiple-floor support structure.

Rotury dryer. A convection rotary dryer, as shown in Fig- ure lb, is a continuous unit with a horizontal, typically slightly inclined rotating cylinder with lifting flights mounted on the inside wall. This unit uses a hot airstream to dry material and can also have a center shaft fitted with plows or paddles that agitate the material and promote heat transfer, in which case the unit is called a rotaryplow dryer orpaddle dryel: The rotary dryer is also available in a con-

duction version; in this unit, rather than using a hot airstream as the only heat input source, the rotating cylin- der is indirectly heated by a heating jacket filled with hot liquid or steam and, sometimes, internal metal tubes and lifting flights filled with hot liquid or steam (in which case the unit is called a steam-tube dryer). The conduction dryer can also operate under vacuum. The dryer can han- dle a range of feeds, including pumpable and non- pumpable semisolids and free-flowing powders and granules. The convection version is frequently used to dry coarse materials that don’t fluidize easily, and the conduc- tion dryer is often used to dry heat-sensitive and low-den- sity materials and to recover solvents.

For the convection dryer’s operation, the feed enters one end of the rotating cylinder and flows in a cocurrent or countercurrent pattern with the hot airstream inside the dryer. Typically, the lifting flights along the dryer wall lift and cascade the material to improve contact with the hot air and speed drying. In the conduction dryer, the feed en- ters one end of the cylinder and is dried as heat transfers from the metal tubes through the dryer wall to the parti- cles. Both dryers provide processing rates from 50 to sev- eral thousand pounds per hour. Residence times for the dryer vary widely depending on the dryer type and size. The rotary dryer’s horizontal configuration can require a lot of floor space.

Fluid-bed dryer. A fluid-bed dryer can be batch or contin- uous. The dryer, as shown in Figure IC, has a vertical or horizontal housing and dries material by convection, send- ing a hot airstream through a perforated plate under a bed of material. It commonly dries free-flowing particles in the pharmaceutical industries (in batch mode) and chemical industries (in continuous mode).

In operation, hot air flows upward through the material on the perforated plate, fluidizing - that is, lifting and agitat- ing -the material as the hot air surrounds each particle, rapidly transferring heat to it. The dryer can have a resi- dence time up to 20 minutes. Longer residence times can be achieved by installing baffle plates or weirs in the dryer. As the material bed flows over the plates or weirs, it turns over, exposing particles under the bed surface to the hot airstream and preventing overdrying of particles that oth- erwise would remain on the surface.

The fluid-bed dryer can process materials at from 50 to several thousand pounds per hour. Its space requirements depend on whether it has a vertical or horizontal configu- ration. Considering your desired final particle size is im- portant when using this dryer because it requires high airflow volumes and must fluidize the feed, thus poten- tially producing excessive fines during drying. In some cases, the dryer is used as a classifier, with fines captured by a dust collector and coarse particles discharging to the downstream process.

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48 Powder and Bulk Engineering, April 2001

Belt conveyor dryer. The belt conveyor dryer, also called a conveyor dryel; is a continuous convection dryer that has a long tunnel-shaped housing and one or more moving mesh or perforated belts (or trays) that carry the feed through the hot airstream in the dryer. The dryer is com- monly used to dry pumpable and nonpumpable semisolid feeds and thick slurries and solutions that have been pre- formed into other shapes, such as briquets. Other common applications are food products such as cereals and french fries.

In operation, the material moves on the belt from one end of the dryer to the other, typically in one pass, while hot air typically flows upward through the moving belt. The air’s temperature varies from one zone in the housing to the next to prevent overdrying. The belt conveyor dryer processes materials at rates from 50 to several thousand pounds per hour. Residence time can be up to about 1 hour, and the dryer requires a lot of floor space.

One form of convection drying is particle dying, in which the hot air moves in and around the individual particles.

Tray dryer. The tray dryer (also called a shelfdryer), as shown in Figure Id, is a batch convection or conduction dryer that consists of a housing into which shelves or trays of material are manually loaded. The unit can also operate under vacuum to dry a heat-sensitive material at a low temperature. (In another version, called a tunnel-truck dryer; a rack of trays loaded with material is pulled through a heated tunnel-like housing.) The tray dryer can handle feed in any form and is commonly used to dry chemicals and pharmaceuticals.

In the convection tray dryer’s operation, a hot airstream circulates around the shelves or trays to directly heat the material; in the conduction version, the shelves or housing walls are heated to transfer heat indirectly to the material. The material bed remains static, with no mechanical mix- ing, so the residence time can be quite lengthy - from several hours to several days, depending on the material, its allowable drying temperature, and the final product’s desired moisture content. As a result, the dryer typically processes materials at rates of just a few hundred pounds per hour with residence times commonly 24 hours or more, making the unit impractical for high production rates. The tray dryer is inexpensive and requires little floor space (except for the tunnel-truck version), but the dryer’s manual loading and unloading makes it labor-intensive.

Rotary tray dryer. The rotary tray dryer’ (also called the Turbo-tray dryer) is a continuous version of the convec-

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rotating circular trays. The rotary tray dryer handles pumpable and nonpumpable semisolids and free-flowing feeds and is commonly used to dry chemicals, pharmaceu- ticals, pigments, polymers, and food ingredients.

In operation, the material is fed onto the dryer’s top tray. After one tray revolution, the material is swept by a sta-

is mixed, leveled by a stationary leveler bar, and then after one revolution is swept by another stationary wiper to the tray below. Fans located in the trays’ center circulate hot air through the housing and over the material, directly

one tray down to the next until the dry final product exits

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The dryer provides gentle material handling and can pro- vide different temperature zones for applications requiring precise temperature control. The dryer’s processing rates

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vary from 50 to several thousand pounds per hour. The res- idence time can vary widely, depending on the number of trays, tray rotation speed, and fan adjustment, and can be quite lengthy. The dryer is often used as the last dryer in a multiple-stage drying process because the long residence time can bring materials to a bone-dry state. The long resi- dence time also allows the dryer to perform chemical reac- tions, to remove bound moisture from a material, and to heat-treat a material. The dryer’s vertical configuration can save floor space.

Conical dryer. Also called a conical mixer-dryer or Nauta dryel; the conical dryer, as shown in Figure 2a, is a batch conduction unit that typically operates under full vacuum. The vertical vessel is cone-shaped and contains an internal rotating screw. The conical dryer commonly dries chemi- cals and pharmaceuticals in nonpumpable semisolid and free-flowing forms.

In operation, the screw inside the dryer vessel rotates and moves around the vessel’s interior to agitate the material and improve mass transfer as the heat is indirectly trans- ferred from the dryer walls to the material. The screw can be heated to reduce the material’s residence time, which typically ranges from 2 to 24 hours. The screw can be top- or bottom-mounted.’ The bottom-mounted screw avoids the placement of the screw’s internal drive components in- side the dryer vessel, which allows the vessel to be com- pletely flooded during washdown for easier cleaning. The conical dryer doesn’t require much floor space but re- quires relatively greater headroom than a dryer with a hor- izontal vessel.

Drum dryer. A drum dryer (also called afilm-drum dryer) is a continuous conduction dryer consisting of one or two rotat- ing heated drums. The dryer processes chemicals in liquid or

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pumpable semisolid form, but is typically not well-suited to drying heat-sensitive or excessively dusty materials.

Powder and Bulk Engineering, April 2001

In operation, a feed device consisting of applicator rolls or spray nozzles applies the feed liquid to each dryer drum as it rotates. In less than one drum rotation, the heat trans- ferred indirectly from the drums to the feed dries the mater- ial, which is then scraped off by a blade mounted adjacent to the drum. The dryer’s operation produces a short resi- dence time, up to a few minutes, and a processing rate of up to several thousand pounds per hour. The dryer can require a lot of floor space, depending on its configuration.

Plate dryer. A plate dryer,’ as shown in Figure 2b, is a con- tinuous version of a conduction tray dryer that consists of a set of heated, stationary plates stacked in a housing and a central rotating shaft equipped with plows and arms. The dryer is typically atmospheric but is available in a vacuum version for drying heat-sensitive materials. Common ap- plications include specialty chemicals, pharmaceuticals, foods, polymers, and pigments. The dryer can handle feeds in pumpable and nonpumpable semisolid form and free-flowing powder or granular form.

Your production rate will determine whether you need a batch or continuous dryer.

In operation, material flows onto the top plate as the rotat- ing shaft causes the arms to move across the plate and push the material across it. The plows continually turn the mate- rial over to improve the heated plate’s indirect heat transfer to the particles. The material then falls from an opening in the plate to the next plate, and so on until the dried material exits the bottom discharge.

The dryer’s processing rates range from 50 to 5,000 lbh, and the residence time can range from 5 to 120 minutes. The long residence time enables the dryer to bring materials to a bone-dry state, so the dryer is often used as the last dryer in a multiple-stage drying process. The long residence time also allows the dryer to perform chemical reactions, to remove bound moisture, and to heat-treat a material, such as by dri- ving off organics from molybdenum ore.

Dryer selection factors Now that you’re familiar with how different dryers work, it’s probably clear to you that many of them aren’t suited to your application. A closer look at your process parameters will help you eliminate many dryers from consideration.

process only 50 kilograms of a fine chemical a day, you’ll need a batch dryer, but to dry 5,000 k g h of the chemical, you’ll require a continuous dryer.

Your feed consistency. The feed’s consistency - whether it’s a slurry or a fine powder - will eliminate several dry- ers from your consideration. Again, refer to Table I1 for in- formation on which dryers can handle your feed.

Your feed’s allowable drying temperature. Some dryers are better than others for drying a heat-sensitive material. For instance, a heat-sensitive pharmaceutical powder may require drying at temperatures between 50°C and 90”C,

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Yourproduction rate. Your production rate will determine whether you need a batch or continuous dryer. If you will

52 Powder and Bulk Engineering, April 2001

which may best be accomplished in a dryer that can oper- ate under vacuum to reduce the residence time and the temperature at which the liquid evaporates.

Your feed's moisture type and quantity. The moisture in your feed can be any of several types, including surface

I Typical dry

1. PRODUCT formula, trade name

2. DRYER CAPACITY 2.1 Wet product feed - Ib/h 8 - % moisture 2.2 Dry prod. discharge - Ib/h 8 - % moisture

Evaporated liquid - Ib/h Bone dry product - Ib/h

WET PRODUCT FEED Originates from : filter centrifuge other Feed; continuous batch other Volume/batch -gal. Batch cycle -min. Consistency: liquid sticky pasty friable thixotropic

What conveyors worked satisfactorily? other

Particle size analysis

Wet product temperature - Compositions of product - moisture (vapor pressure -

Moisture determination: -Drying chamber -

curves?) -

constant weight - under vacuum -

Wet bulk density (loose) __ Dry bulk density (loose) - True product bulk density - way of measuring the bulk densitp

- other

Melting Point -OF Spec. heat wet product __ Spec. heat solids - Spec. heat moisture - ph value - Max. admissible wet product temp. - Dryer atmosphere: air, inert, vacuum

"F Yo - % __ % -

"F min.

in Hg

IbsM Ibsm IbsM

BTUIlb BTU/lb BTU/lb

O F

moisture, interstitial moisture, and bound moisture. Dif- ferent dryers are suited to removing different types of moisture. For instance, a flash or fluid-bed dryer can rapidly remove large quantities of surface moisture, but wouldn't be able to remove bound moisture, which can re- quire a dryer that operates with both a high temperature and long residence time.

we 3

4.1 4.2 4.3 4.4

4.5 4.6 4.7 4.8 4.9

Requested form of dry product Admissible dry product temp. -OF Dry product to be cooled y e s -no to -OF Is there some isothermal absorption known of the dry product? (Please enclose) - yes -no Electrostatic charging -yes -no Risk of oxidation -yes -no Is the product hygroscopic? -yes - no Melting point - "F Convey ability

5. Do you dry the product in your plant now? What dryer type? Wah what results?

- 6. 7. Is exolosion ventina reauired? no ves For solvent? 1

be used? For contact w/wet product For contact w/dry product For contact w/fresh gas For contact wlexhaust gas

- 9.

PLEASE ENCLOSE SAFETY DAT TO HAZARDOUS PROPERTIES (TOXIC, IGNITABLE, INFLAMMABLE, ETC.)

10. AMBIENT AIR CONDITIONS 10.1 Temperature -OF 10.2 Humidity __ 70 or __ Lbs. H,O/lb. Dry Air

- 11. 11.1

11.2 Heating Media: Steam ~ @ - PSlG OIL ___ 8 - BTU/lb GAS 8 BTUIfP

Your feed’s water or solvent content. Any dryer can re- move water from a feed, but some, such as a spray dryer or belt conveyor dryer, can’t safely remove a hazardous sol- vent. If your feed is wet with a hazardous solvent, your dryer choices will be more limited.

Yourfeed’sparticle size. The dryer must be able to handle your feed particles - whether they’re submicron size or as large as golf balls.

Yourfinal product’s toxic or hazardous nature. If your final product will be toxic or hazardous, the dryer must have airtight construction. This eliminates some dryers - such as a belt conveyor dryer without an airtight housing -from consideration.

Once you’ve used these selection factors to narrow your dryer choices, work with a dryer manufacturer to determine which dryer is best suited to your application. To help you choose a dryer, the manufacturer will ask you to complete a dryer questionnaire, such as the one shownin Figure 3, with detailed information about your feed, processing parame- ters, and desired final product characteristics. PBE

Powder and Bulk Engineering, April 2001 53

Endnote 1. All equipment described in this article is made by more

than one manufacturer except these three units: the ro- tary tray dryer, which is made by Wyssmont Co., Fort Lee, N.J., the version of the conical mixer with a bot- tom-mounted screw and the plate dryer, which are both made by Krauss-Maffei Process Technology, Florence, KY.

For further reading Find more information on dryers in articles listed under “Drying” in Powder and Bulk Engineering’s comprehen- sive “Index to articles” (in the December 2000 issue and at www.powderbulk.com).

Patrick McCarty is product managerfor dryers at Krauss- Maffei Process Technology, PO Box 6270, Florence, KY 41 042; 859-283-4335, fax 859-283-1 878 (pmccarty @kmpt.net). He holds a BS in chemistryfiom Wichita State University in Kansas.

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