I:

rying is often called as much an art as a science. I t is a complex D opera tion, involriiig simulta-

neous heat and mass transfer. In addi- tion, the presence of solids affects heat, liquid and vapor movement, retarding both transfer processes, particularly during the final drying stages, or when the solids phase is continuous. Knowl- edge, judgment and careful testing are essential to dryer design. lJnlike in the case of some unit operations, calcula- tions alone are not sufficient for com- ing up with a successful install a t’ ion.

The key areas of concern in design- ing a drying system are: materials-han- Giiilg c L ~ i i ~ ~ y , safety and fitness to meet product specifications. Of the three, handling capacity is often paid least at- tention to, despite being the critical de- :;ign factor. once safety xiid product specifications are met. This article, em- phasizing handling capacities of dryers, can help you select the right unit for your needs.

Lri selectin? a dryer, the first consid- eration is, of tourse, safety. Questions

THE KEYTO

Here is how solids can be moved in and out of a unit without problems of sticking and plugging. Direct

and indirect dryers are discussed.

Paul Y. McCormick, DRYING Unincorporated

that naturally arise include: Can Ihi-1 wet material be sent through the drqer without releasing toxic or dangerous fumes? Without catching fire or ex- ploding? Without endangering tlw health and safety of ol,erators? ‘This is an extensive topic, and space docs iiot permit discussioii here. Some rcreni, technical articles in Chewica I I ’ x ~ L - neenng should be helpiiil 7,vit.h reigarci to safety:

Coverage of the Intcrnational S j m-

posiuin on Preventing Major Chemical Accidents, Mar. 16, 1987, p. 14.

Flammability of low-Btu gases, John J. Clawsnn, Aug. 17, 1987, p. 169.

e Hazard identification, analysis and control, Henry Ozog, Feb. 18, 1985, p. 161.

0 Eliminating potential process haz- ards - report, Trevor A. Kletz, Apr. 1, 1965, 11. 48.

The next consideration is: Will the dl.yer meet the product specifications for moisture, type of particle, etc.?

This consideration should be obvious, and addressed by engineers in design- ing n new drying system. On the other ham, the qucistion: Will the dryer han- dle the niaterials? is often assumed not to he a problem a t all. However, this is a h ays a critical design question, once p;ifety and product specs are met.

Materials handling is vital W h y i, mirtwials handling a major con-

11 tor dryers? For example, consider a neir plant: is dryer that is mable to liandle its rated capacity or that cannot

produce the desired product-moisture content is certainly an embarrassment; but if the wet material can be coaxed through it somehow, the situation can eventually be remedied. Perhaps, the system needs more gas-handling or heating capacity, or a section added to the dryer, or even a second-stage unit. Nonetheless, if the dryer can be per- suaded to run even a t a reduced rate, the plant operates. But if the solids can- not be moved successfully through the dryer, the plant may never operate.

Furthermore, the drying usually takes place near the end of the process, and is often followed only by screening, blending and packaging. Thus, the dry- er must accommodate any off-specifi- cation and other unusual material that upstream operations send to it. These include unreacted feeds, raw-material

ples), more air is needed to remove va- por than to heat the product. Now, the same type dryers, with different inlet- gas temperatures, can be compared to show the advantage of using a higher inlet-gas temperature.

Example 1: Given: The ambient tem- perature is 20°C. The heat to take the water from 20 to 100°C and then vapor- ize it is 2,590 kJ/kg. The average specif- ic heat of air a t 100-1.000"C is 1.1 kJ/

In selecting dryers, consider how each type handles materials. Solids must not be too difficult to move through a particular unit.

impurities, ultrafine crystals that can- not be properly dewatered and become dust when dried, etc.

During startup, if a continuous dryer chokes because of such problems, the unit must be stopped and cleaned. If there is no upstream holdup available, the process shuts down. A baffling situ- ation is created Something in the pro- cess upsets the dryer, but the process cannot run long enough to isolate the problem and correct it before the dryer chokes. The report to the manager's of- fice states simply: The dryer won't work.

First, bear in mind that there are nu- merous types of dryers, which can be classified in several ways [2,.4,11,13]. A convenient one is by type of heat transfer:

1. Direct dryers - convection from a

(kg)("C). Assume that 3.0 kg of air are supplied to a dryer per 1.0 kg of water evaporated, that the air leaves a t 100"C, and that 10% margin is allowed for solids sensible-heat gain and ambient losses. Then, air must enter at: [2,590/ (1.1 x 3.0 x 0.9)] + 100 = 972°C.

The exiting air has a dewpoint of 73°C and a humid volume of 1.6 m3/kg ofdry air; it contains 0.33 kg of moisturelkg of dry air above ambient humidity akd is about 35% saturated (these, and oth- er values are from psychrometric ta- bles). Thus, exit volume is 4.8 m3/kg of water evaporated. The evaporation ef-

ately: [2,590/(1.1 x 0.82, which is good. s with a similar *

lower inlet-gas temperature: le 2 Given: Ambient s again 20°C. The heat needed

to raise water from 20 to 100°C and va- re. The average spe-

(kg)("C). In this ex air enters the dryer a t 500°C and leaves a t 100°C. Again, allow 10% for sensible heat and heat losses. Then, the air re- quired per 1.0 kg of water evaporated is: 2,590/[1.05 x (500 - 100) X 0.91 = 7.0 kg. Air leaving the dryer carries 0.14 kg of water vapor/kg of dry air, above ambient humidity. Its dewpoint is 60°C and humid volume is 1.3 m3/kg of dry air; thus, exit volume is 9.1 m3/ kg of water evaporated. Evaporation efficiency is approximately equal to

ERINGlAUGUST 15,1988

hot gas contacting the wet material. 2. Indirect (also called contact-heat-

ing) dryers-conduction from a hot surface contacting the material.

3. Radiant dryers -radiation from a hot gas or hot surface that contacts, or is within sight of, the material.

4. Dielectric (or radio-frequency) and microwave dryers - using high-fre- quency electric fields or microwaves, respectively.

Many dryers employ more than one heat-transfer mode; however, most in- dustrial dryers are characterized by one that predominates.

ing, see the box. Guidelines for stop- ping heat losses will appear in the Sept. 12,1988, Plant Notebook - space does not allow inclusion of this topic in this article.

For tips on dryer selection

[2,590/(1.05 x (500 - 20) x 7.0)] = 0.73. Dust recovery for Example 2 costs

inlet-gas temperatures reduce fuel us- age. However, the big payoff comes from reducing the investment in dust-

Note that the thick almost the same fo

secondary, dilution gas emitted to

environment. Nevertheless, recirculat- ed gas must be cleaned (its solids must be removed); otherwise, the dryer's heater fouls with dust. The cost of dust collection is high: For the dryer in Ex- ample 1, with a typical inlet tempera- ture of 900-1,000"C, dust collection rep- resents 50% of the total dryer investment. Yearly maintenance costs can exceed half of this 50%.

Now, let us consider some common direct dryers in more detail:

A simple pneumatic-conveyor flash dryer may employ a venturi to

The conveying-tube gas velocity is usually 25-35 m/s, while that in the venturi throat, which is one-half the tube diameter, is 100-140 m/s. The re- duced throat accelerates and disperses the feed solids by momentum transfer from the high-velocity gas stream; the diffuser recovers static-pressure head from the velocity head, and minimizes blower power demand.

The s'tatic pressure in the feed nozzle above the throat is kept close to atmo- spheric to minimize air inleakage; thus, the throat gage pressure a t that point is 0. If only a push blower can be used,

hen necessary to recover enough pressure in the diffuser to over- all flow resistances downstream. velocities up to 50 m/s are used to

improve solids dispersion, but a t a high cost in blower power.

Most of the drying work is done in the throat, diffuser and first short-tube section where there are substantial ve- locity differences between gas and sol- ids. Once solids reach about 80% of the gas velocity (about as high as they reach on the average), drying is mar- ginal. The drying process is like that in a spray dryer where, except in the at- omizer zone of high droplet velocities,

le drying proceeds under essen- stagnant conditions a t a particle

olds number approaching 0 (the article velocity relative to the gas ve- city is 0), and a Nusselt number of 2, hich indicates stagnant conditions. Obviously, if such an arrangement is

to dry at 211, wet solids fed ints the

throat must instantly disperse as indi- Although these dryers evaporate vidual particles in the gas stream, so large quantities of surface liquid, limit- that all liquid is exposed on particle sur- ed solids holdup makes them unsuit- faces: Effective holdup for drying is able for removing capillary and bound only two or three seconds. liquid. Thus, pneumatic-conveyor dry-

Jn some pneumatic-conveyor flash ers complement, rather than compete dryers, the feed is dispersed by a pad- with, rotary and fluidized-bed dryers. dle or knife wheel, similar to the impel- Many are employed as first-stage dry- ler of a materials-handling fan. The ers preceding the longer-holdup dryers feed often should be conditioned -typi- for the manufacture of polyethylene, cally, by recycling dry material -so polypropylene and other resins. that even while wet, it does not stick to Holdup may be increased by (1) in- the tube. Unlike with a spray dryer, serting sections in the conveying tubes there is no chance that all particles will to slow solids flow; (2) imparting spiral dry before they contact the wall. flow patterns in these tubes; and (3)

These dryers may also include an in- providing closed-loop conveying tubes line solids classifier to separate coarse with internal classifiers so that only particles, which may be damp, from dry fines, presumably dry, can escape. fines. Coarse particles return from the Pneumatic-conveyer flash dryers are classifier to a feed-conditioning mixer compact and install more conveniently where some recycled dry product may than rotary, fluidized-bed or belt types. also be added. Fines pass to product-re- They have few moving parts, which covery cyclones. makes them adaptable for recovering

FIGURE 2. The gas-solids contacting in fluidized beds has a more-eff ective surface and highs; heat- siid i?iass transfer than in aireci rotary dryers

solvent with inert gases. This is be- cause no rotating seals are needed- these require packing, which often leaks. However, pneumatic conveyors are, in effect, low-power fluid energy mills; if crystal breakage is undesir- able, avoid this type of dryer. Let us now compare direct-rotary and fluid- ized-bed dryers.

The illustration on the first page of this section shows a directirotary dr er. Wet solids are lifted by flights and showered through the gas stream, which moves counter- or cocurrently to the gas. Shell knockers discourage ad- hesion to metal surfaces. Solids are moved through the cylinder, which is up to 3 m dia. and 30 m long, by the ro- tation plus a slight incline in the solids- flow direction.

In a fluidized-bed, on the other hand, solids movement results solely from the gas flow through the bed. In Fig. la, solids move near a distributor- plate perforation in a dilute, shallow bed, illustrating how eddy currents draw solids across the plate. This mech- anism causes buildup around a perfora- tion when drying sticky or low-melting solids and using high inlet-gas tempera- tures. The plate surface-temperature near the hole approaches the plenum gas temperature. Such a buildup can impair flow and heat-transfer. Thus, not all types of solids are well-suited for the fluidized bed.

In dense-phase fluidized beds, enter- ing gas typically forms a bubble above the perforation before it separates to rise through the solids, much like air acts as it escapes from a small leak in an inner tube underwater (see Fig. lb).

Fig. 2 compares the efficiency of heat- and mass transfer in fluidized beds with that in rotary dryers; effi- ciency is, of course, affected by solids surface exposure. In the rotary dryer, gas passes between fairly well-defined parallel “curtains” that move across the pat’ Jf gas flow as the cylinder ro-

- tates. Turbulence allows some gas to penetrate the curtains. Also, by in- creasing the cylinder’s rotational speed, solids dispersion is enhanced; yet mixing remains comparatively poor. In the fluidized bed, there is a well-dispersed cloud of individual parti- cles; surface exposure is maximized.

Because of this difference in gas-sol- ids contacting quality, the volumetric

Y-

~

\ Fluidized-bed heaters have much higher \\

heat-transfer coefficients than do direct, rotary dryers and are often preferred.

FIGURE 4. In this two-stage vertichl fluidized-bed dryer, a significant pressure drop is needed for proper functioning

116 CHEMICAL ENGINEERING/AUGUST 15,1988 *i 1,

heat-transfer coefficient in the rotary dryer is about 1% of that realized in the fluidized bed; that is, Ua = about 0.18 kW/(m3)("C) in the rotary dryer vs. about 18 kW/(m3)("C) in the fluidized bed. Specifically, the reason is that so- lids surface-exposure in the fluidized bed is greater; the value of a in the com- pound symbol used to signify the volu- metric coefficient, Ua, is greater; where U [kW/(mz)("C)] is a surface- based heat-transfer coefficient; a [m2/ m3] is the exposed solids surface per unit volume of dryer.

Typical temperature profiles are shown in Fig. 3 for each dryer in single- stage cocurrent flow. The rotary dryer with conventional lifters needs approxi- mately five shell diameters to achieve temperature equilibrium between gas and solids, with the gas entering a t 500°C and the wet solids at 20°C. On the other hand, the fluidized bed may reach temperature equilibrium a fraction of a meter above the gas distributor.

For countercurrent gas-solids flow, the situation is more complex: A verti- cal two-stage fluidized bed (an arrange- ment used for multistage drying), is shown in Fig. 4. This materials-han- dling situation requires care, even when processing solids of good fluidiz- ing character, because a significant pressure drop, 1-2 kPa, is needed to as-

FIPURE 5. A countercurrent direct dryer. A constant temperature-differential helps heat transfer and helps to minimize penalty of cocurrent dryers

sure proper gas distribution in the up- per stage. This pressure drop, in turn, requires that the solids-transfer device between stages also serve as a gas seal.

Alternatives for a gas seal are a flu- idized downcomer (similar to a baro- metric condenser liquid-seal), and an external chute and rotary valve. The

downcomer and chutehalve arrange- ments require free-flowing solids at all stages of heating and drying. Tempera- ture profiles in the figure show each stage at equilibrium, with gas and so- lids leaving a t the same temperature.

Fig. 5 shows a countercurrent ro- tary dryer: Apart from drag resistance

CHEMICAL ENG1N);:EKlNGIAUGUST 15,1988 117

imposed by showering solids, there is no internal cylinder resistance to gas flow; pressure drop end-to-end is about 0.1 kPa. Also, because gas and solids do not reach temperature equilibrium a t any point, a greater temperature driv- ing force, compared to cocurrent flow or that in a fluidized bed, minimizes the penalty of a relatively low heat-trans- fer coefficient. One theoretical stage is about 1.5 shell diameters of length in a countercurrent rotary dryer.

Comparing direct fluidized beds to rotary dryers, for low-temperature dry- ing of consistently fluidizable solids, the fluidized bed is first choice because of superior heat and mass transfer. The key is that the bed must be fluidizable 100% of the time [IO].

The cocurrent rotary dryer is best for low-temperature drying of heat- sensitive materials that do not fluidize dependably and are too sluggish for spouted beds. In cocurrent operation, only the wet solids are exposed to the high-temperature inlet gas. This high temperature is less harmful to the so- lids while wet, since evaporating liquid holds the solids a t the gas wet-bulb temperature. In countercurrent flow, the dry solids contact the hot inleq gas, and , since nearly all of the moisture is

FIGURE 6. Spouted-bed units are an alternative to fluidized-bed dryers

now gone, there is a danger of dam%- 1 ing heat-sensitive materials. Thus, co-

current rotary dryers are best if there are doubts about solids heat sensitivity.

When a product should leave a t much higher temperature than the exit gas, for energy efficiency, as in rotary kilns, a countercurrent rotary dryer is pre- ferred to a fluidized bed because inter- nal-pressure seals between stages for gas and solids are not needed.

To summarize, fluidized-beds are most-attractive for heat and mass transfer, etc., and their use will in- crease as more is learned about their materials-handling capabilities. If there is doubt about materials-handling, rota- ry dryers are still the best choice.

Spouted-Bed dryers (Fig. 6) substi- tute for fluidized beds when materials are too coarse to fluidize a t reasonable velocities. The fluidized bed’s heat- and mass-transfer advantages, and its ma- terials-handling disadvantages, are generally shared by spouted beds.

Applications include drying polymer pellets prior to remelting and extru- sion; but for these pellets, the more simple, purged-hopper dryers use less energy, cause less attrition, and, in con- tinuous operation, allow better plug- flow control. Such devices are basically vertical cylinders with feed moving downward (a cone a t the base controls product flow) and the gas entering through a sparger and moving upward.

and convection

efore looking at specific indirect B dryers, a question needs to be an- swered: Why use indirect heat?

There can be an economic advantage. Gas flow is needed only to remove va- pors. A relatively low flowrate of purge gas (in countercurrent flow) is required to discharge vapor-laden purge gas, typically, a t 90°C and 90% saturation, with a humidity of 1.0 kg/kg of dry air. Only a small fraction of total heat input is lost in this purge gas, since flow is relatively low.

On fhe other hand, owing to the high saturation of the exit air, dry collectors cannot be used, and wet scrubbers are

employed. Although these devices are about one-third the size of the dry, fab- ric collectors used for direct drying, the latter offer fewer operating problems and are easier to maintain. Also, dry collectors require much less pressure drop for efficient collection of fines. This favors direct dryers.

An important long-term advantage of indirect dryers is that a number of energy sources can be used (including steam generated in coal-fired boilers, heat-transfer fluids, etc.), while direct

units usually use light fuel-oil or natu- ral gas. I t is often impossible to find other energy sources for economical di- rect heat. In a fuel crisis, indirect dry- ers are less likely to be affected by the use of alternative fuels. Let us now cov- er specific types of indirect units:

Paddle-agitator dryers (Fig. 7) are used in either direct or indirect service. As direct dryers, their discussion would follow spouted beds in the previous sec- tion. The indirect setup is illustrated, and so they are listed here. For direct

FIGURE 7. Used in direct or indirect service, this paddle-agitator dryer offers longer holdup (up to 1 min) by varying the paddle angle

CHEMICAI, ENGINEERING/AUGUST 15, 1 W 119 L

Indirect fluidized-bed dryers are used when solvent recovery or a n inert purge gas is required. Maintenance costs are relatively low.

FIGURE 8. Indirect fluidized-bed dryer is replacing the steam-tube, especially for solvent recovery

FIGURE 9. This rotating, indirect spiral dryer has a long solids holdup

120 CHEMICAL ENGINEERINGIAUGUST 15,1958

drying, without a jacket, the dryer is in- stalled as a feed section in a pneumatic conveyor, replacing a venturi, paddle wheel or mill.

By adjusting paddle pitch to retard solids flow, counter to gas flow,\.solids holdup can be increased up to one min- ute. For agglomerated feeds requiring partial drying, mechanical deagglomer- ation and more drying, this setup can eliminate the need for second- and third-stage drying, with substantial in- vestment and operating cost savings.

The lead illustration of this section shows a steam-tube rotary dryer. Like a direct rotary dryer, this unit is versatile in handling materials. The surface is relatively inexpensive pipe; a dryer may contain 1,500 m2 of heating area and evaporate 7,500 kglh of water.

The usual steam pressure is 1,000 kPa, but dryers are built for 3,500 kPa for calcining. Purge gas flows counter- currently to the material, at a rate of about 1.0 kg/kg of water evaporated, to sweep vapors away from the dry end. Occasionally, a dryer is reported operating without a purge gas. How- ever, the usual situation is that the unit has many leaks. Thus, purge flow is un- controlled and probably is excessive.

Overall, heat transfer by conduction and radiation are: 30-60 W/(mz))("C) for

organic solids; and 60-90 W/(mz)("C) for inorganic salts, oxides and minerals, a t 1,000-kPa tube pressure and based on total tube surface, even though no more than 2530% of this surface is submerged at any instant. (Convection is insignificant in contact heating.)

When the cylinder diameter exceeds 2 m, three tube rows usually are provid- ed. It is essential that material not stick and pack behied the tubes. Cylinder knockers are ineffective and there is no nondestructive method of vibrating the tubes; thus, some dry product is often recycled.

Backmixing is probably the best pro- tection when the quality of the wet feed is uncertain. If the dryer can convey dry product, operation on almost 100% liquid feed is possible.

The recycle hopper above the recycle mixer is always filled to allow for un- scheduled shutdowns. Also, storage sufficient for one dryer fill is needed to start up an empty dryer; the first dry product replenishes the hopper.

Fig. 8 is an indirect fluidized bed, which is becoming an increasingly pop- ular substitute for the steam-tube rota- ry diyer, especially in processes where solvent is recovered or an inert purge gas is required: Although rotary dryers can be supplied with gas-tight seals, '

these seals are very expensive, compli-

cated and require constant mainte- nance. Also, the rotary dryer itself must be built with tighter tolerances than are needed for fluidized beds. A process vessel that does not rotate is better for inert-gas circulation.

The fluidized-bed heating surface usually is a vertical plate coil. The plate coils are spaced 0.2-0.4 m apart to pre- vent packing of material between rows, which may be 2 m high. Pipe coils are sometimes used instead of plates, but coils are not as good for drying because the pipes interfere with fluidization, and material tends to pile in a stagnant drift on the top of each pipe section.

Plate coils can be arranged in pat- terns that provide material plugflow, backmixing or a mkiure of both. Fluid- izing-gas plenums beneath the distribu- tor that feed the inlet gas into the vari- ous sections of the dryer allow changing gas velocity and temperature between stages. Separate plate-coil banks also permit heating-medium tem- perature changes.

These options give indirect fluidized beds the flexibility for fine-tuning, which is impossible in steam-tube dry- ers. Steam-tube dryers are limited to one heating-surface temperature and one gas rate; also, since there is only one inlet-gas stream, there can be only one inlet temperature.

In indirect fluidized-bed dryers, the fluidizing gas is also the purge gas and contributes little heat for drying, 80- 90% of which comes from conduction and radiation from the heating sur- faces. Because the vessel is stationary, all feed, product, heating-medium, etc., connections are rigid and permanent.

As with most indirect dryers, the flu- idized bed is well-suited for fine pow- ders: A low gas flow in the bed mini- mizes dusting. Recirculating the purge minimizes emissions and recovers sol- vents. External product recycle or in- tel=nal backmixing enhance material fluidization and protect against up- stream process upsets. In sum, the flu- idized bed approaches an ideal for indi- rect-heat drying, provided that the wet feed-solids can be fluidized at all times.

Indirect pneumatic-conveyer dry- ers are formed by jacketing the tube of the direct-heated unit. Two heat-trans- fer mechanisms are involved: First, particles momentarily contact the hot surface: second: the warmed particles

move away from the surface and trans- fer heat to other particles by mixing.

Indirect conveyors are used for inert- gas operations, to minimize the amount of gas used, and for other drying opera- tions where heat loads are not high and all liquid is surface liquid - e.g., poly- mer pellet drying in an oxygen-free at- mosphere after extrusion and water quenching. Indirect heat-transfer to dispersed particles is inefficient com- pared with direct heating, and particle retention time is short.

to-solids heat-transfer coefficient is 150-250 W/(mz)rC). The tube may be lengthened to provide the needed amount of surface for indirect drying; the differential velocity between gas and solids is not a factor in indirect heat-transfer.

A more sophisticated design is shown id Fig. 9. In this spiral (vertical) dry- er, pentrifugal force holds a thin layer of particles against the heated outer tube-wall, while the solids are conveyed upward. The center tube also is heated and rotates slowly to assist particle mixing. The result is longer solids hold- up in a compact dryer. Particles must be free-flowing and nonsticky a t all stages. Obviously, indirect-heat dryers are more sensitive to material sticking on surfaces because adhesion directly interferes with conduction.

For indirect dryers, the effect of agi-

At a gas velocity of 30 m/s

tator speed on heat transfer is given by Root [9]. One of the first high-speed agitator dryers was introduced about 25 years ago, and consists of a tube and internal paddles. For indirect heat, the tube is jacketed and the paddles a re- pitched to hold the solids fill to less than 10% of shell volume. Purge gas usually flows countercurrently, so the paddles are given a slight forward pitch. With paddle tip-speeds of 5-15 m/s, wall-to- solids heat-transfer coefficients are 300-350 W/(m2)(T), five times those in a steam tube dryer and comparable to plate-coil transfer in fluidized-bed dryers.

The high-speed agitator promotes deagglomeration, minimizes wall stick- ing, and combines the high heat trans- fer of a fluidized bed with the solids- handling versatility of the rotary dryer. However, there are disadvantages: par- ticle grinding and crystal breakage. The largest such units evaporate 2,000- 3,000 kg of water/h.

For holdups exceeding one minute, this dryer is generally unsuitable be- cause of filling limits and power de- mand. There is a tradeoff Increase the fill above lo%, but reduce speed and lose heat-transfer efficiency.

Many of these dryers serve as pre- dryers, removing surface moisture ahead of fluidized beds, hopper dryers and heated-agitator dryers that operate a t lower speeds.

FIGURE 10. Heated-disc agitator dryer overcomes many problems of dryer modeled after a ribbon mixer

CHEMICAL ENGINEERING/AUGUST 15,1988 121

but because these dryers usually run a t least half full, and some, like those in Fig. 10, up to 90% full, mechanical stresses and power demands may be in- tolerable at higher speeds, especially when drying sticky and sluggish materials.

tween efficiency and fill, with power usage and mechanical stress limits be- ing the dependent variables. For high fills and long holdup, combined with +e need to accommodate pasty, sticky an? sluggish materials, usually the best choice is a low-speed unit with as much heating surface as possible built the agitator.

Here again, a tradeoff is ne

The batch, rotati bling a portable cement ally employed in vacuum

than for an agitator dryer. The double- cone cannot be used for sticky materi- als or those having soluble fractions.

Finally, for extremely difficult-to- handle solids, Fig. 11 is a plan view of a continuously recirculating heated- screw dryer, the ring-around-the-rosy (RAR) dryer. It is filled with dry prod- uct or an inert material, which is heated to operating temperature by the recir- culating heated screws. Feed, usually a sludge, solution, slurry or pumpable mud, is introduced at a rate relative to the recirculating solids rate, so that the

solids’ sensible heat is sufficient to dry the feed to a nonstick condition before it contacts heated metal.

The screws must be kept clean by contacting only dry product. Heat transfer is by conduction and radiation from the screws to the dry solids, then, by mixing and conduction, to the wet feed. Recirculation rates can be 1,OOO:l.

Depending on particle size and spe- cific gravity, overall heat-transfer coef- ficients are 50-100 W/(m2))(“C) - rela- tively low, but explained by the fact that there are no wetted heating sur- faces. Heated screws, or heated paddle

ay be 0.5 m dia., 6 m long. ed units that have recircu-

lating inert-gas systems safely handle toxic, flammable, etc.

References 1. Ashworth, J. C., ed., “Proceedings of the

Third International Drying Symposium,” Drying Research Ltd., Wolverhampton, U.K., 1982.

2. Dittman, F. W., Chem. Eng., Vol. 84: No. 2, pp. 106-108, Jan. 17, 1977.

3. Hall, C. W., ed , Dryin Technology- An International Journat Vol. 1-5, Marcel Dekker, New York, 1983-1987.

duction to Industrial Dry- ergamon Press, Elmsford,

ed., “Proceedings of the First International Symposium on Drymg,” Science Press, Princeton, N.J., 1978.

6. Mujumdar, A. S., ed., “Drying ‘80, ‘84, ‘86,” (three books) Hemis here Publishing Co., Washington, D.C., 19&-1986.

van’t Land, C. M.,

The aut Paul Y. McCormick is principal engineer of DRYING Unincorpo- rated, 215 Hullihen Dr., Newark, DE 19711. Tel: (302) 731-4736. The company engineers dryers, kilns, ovens and related processing equipment. Formerly, McCor- mick was employed 111 similar work in the En ineering Dept. of Dupont Co. He%as written sever- al papers on drying, and chapters in editions of “Perry’s Chemical Engmeers Handbook,” “The Encyclopedia of Chemical Technology,” and “The Encyclopedia of Polymer Science and Engineer- ing. ’ McCormick is a 1944 raduate of S racuse Uniyersity, a member of AyChE and a lcensed engineer.