Handbook of Industrial Drying

particle size; (2) a bility to han dle pol ydisperse sol^hi ^od^ls^l;^{ow heaters (paddles) without using gas as a heat-}

2. 
   ability to operate at low aeration rates and henⁱⁿce^{g medium. The paddles revolve at a low speed (10 to} lower pressure drops; (4) gen tle ha ndling of pro du⁴c⁰t^r;^{pm) inside the grooved trough fitted with a jacket}
   

(5) higher heat trans fer and drying rates, and othe⁽r^Fs ⁱ.^{gur e 41.1) .}
Since the equipment is mo unted on resonan ce spring s,^{The heating medium passes inside the hollow pad-} _{the power co nsumpt ion for vibrat ion is mini mal} dl_fe_orso that the entire surface of the paddles and shafts

well-d esigne d VFB [6].
acts as the heat transfer surface. The cuneiform blade

The vibration vector is typicall y a pplied at a sm^enal^hal ^{nces agitation of the material and at the same} angle to the vertical to permit conveying of the soli^td^ims^{e prevents the powder from adhering to the heat} _{in the long direction at the desir ed rate. This perm}tr_ia_tn_ssfer surface. For greatest heating efficiency the control of resi dence times an d also be tter con tro^dl^ryo^ef^{r is tilted slightly in the direction of product}

the drying or heati ng rates as the material progres^fls^oe^ws
and is designed so that the material contacts all

downst ream.

4. 
   .6 Cont act Flui d-Bed Dryers
   

heated surfaces, both front and back. The wet prod-
uct is fed continuously at the top of the dryer at one end. As the powder is agitated slowly by the heated rotating paddles, the moisture generated is conveyed

Conta ct FB units are ch aracterize d by the reside^on^uce^{t by a flow of hot air or other gas.}
time distribut ion of the individ ual pa rticles insi de the^{The main features of the paddle dryer are: (1) it is} unit. A broad resi dence time dist ribution is obtain^ce^od^{mpact; (2) has high heat transfer coefficient and} _{in a back -mixed FB in which the lengt h/width rati} g_ooo_od_f thermal efficiency; (3) the paddles have an inter- the be d is relat ively small. The na rrow residen ce ^pt^li^am^ye^{for self-cleaning; (4) it is easy to control; and} _{distribut ion is obtaine d in a plug-flow FB in wh}(_i5_c)_ha small amount of gas is required that minimizes the lengt h/widt h ratio of FB is very large . This ^dc^uo^sr^ti-^{ng and other problems.}
responds to a long, narrow FB. Alternat ively, this can ^{Paddle dryers have been successfully used in dry-}

be obt ained by compart menta lizing FB and is usual practice followe d in the indust ry.
i_tn_hg_e such polymers as VC resin, nylon pellets, and
polypropylene (PP), as well as polyethylene. Operated

_{Compared with the plug-flow FB, a back-mixed FB}in a closed-cycle mode they can recover organics from
has a significant advantage inasmuch as the back-mixed^{such solvent-laden products as polyethylene or PP} FB can accept a feed material that is not readily fluid^aniz^d- ^{can reduce the air volume requirement to only} able. This is possible owing to the vigorous mixin⁵g^{to 10% of that used in direct dryers.}
inside FB and that the material inside the bed acts as a^{Energy requirements for such dryers are also}
large reservoir in which incoming feed material will b^loe^{wer. It is seen that 1300 to 1500 Btu is required to} dispersed and the surface moisture will be flashed of^df^r,^{y 1 lb of moisture with the paddle dryer compared} making the product fluidizable. This characteristic ^{with 3000 Btu/lb for a suspended air unit. Because of} makes the back-mixed FB concept well suited as th^the^{e smaller air volume needed, the sizes of down-}

predrying stage in many polymer-drying systems.
stream condensers and refrigeration system units are

The plug-flow FB drying con cept is parti cular^rly^educed.
suitable for drying bound mois ture from heat-sensi tive
material s since the resi dence time is control led wi ₄th₁i_.n_{3.4.8 Plate Dryer}
narrow lim its. In the typical polyme r applic ation, this
means that the bound mois ture can be remove d frTohme plate dryer (PD) is an indirect dryer in which heat the polyme r product at the low est possible produtractnsfer is accomplished by conduction between the
tempe rature. heated plate surface and the product. It comes under
Thes e two concepts, along with heating of the btehdree major variations, e.g., atmospheric, gas tight,
indirec tly by imm ersed heat exchange surfa ces, aarned vacuum.
jointly util ized in co ntact FBD. In FB applic ationsIn these dryers, the product to be dried is metered

for polymers with indirect heating, the temperature of
the heating panels is typically limited by the softening point of the polymer.
and continuously fed onto the top plate. A vertical
rotating shaft provided with radial arms and self- aligning plows conveys the product in a spiral pattern

FIGURE 41.1 Paddle dryer.

across stationar y plate s. The plates are heated bygroaup of plate s may be heated or cooled indivi dually , liquid medium or steam. Small plate s with internthaul s offer ing precis e con trol of the prod uct tempe ra- rims an d large plate s with exter nal rim s are arratnugre dan d the possibili ty of adjust ing a tempe rature

in an alte rnating sequ ence (Figure 41.2) . profile dur ing the drying process . Thermal deg rad-
Thi s arrange ment makes the product drop froamtion of sensitiv e mate rials can thus be avoided, and
the outsi de edge of the small plate down to the lcaorgoeling subsequent to drying can be achieve d.
plate, on which it is co nveyed to the insid e ed ge, anInd the plate dryer, the produ ct layer is kept shal-
then drops to the foll owing smaller plate, where itlowis (approxi mate ly 10 mm). The entir e plate surface
conveyed again toward the ex ternal edge. This de siigsn utilized for heat trans fer. The product su rface ex-
of the conveying system en sures plug flow of tphoesed to the surround ing atmos phere is even large r produc t throu ghout the entir e dryer. Each plate thoarn the actual ‘‘wetted’’ heat exchange surfa ce. The
design of the pro duct-c onveying syst em ensures pro d- uct turnover numbers in the range of 200 to 1500. A thin prod uct layer on a large heat exchan ge surface coupled wi th high product turnover impr oves both
heat an d mass transfer rates. Fr om va cuum plate

1. 
   Product
   

2. 
   Heating or cooling medium
   
3. 
   Shell
   

housing
dryers , the evaporat ed volatile s are remove d by evacuat ion. Solvent s ca n be recover ed economic ally by simple conden sation [8].
Plate dryers are typicall y fabri cated in a mod ular design; this yiel ds a wide range of dryer sizes with a

4. 
   Conveying system
   
5. 
   Plate
   

heat exc hange surface betw een 3.8 and

41.3.4.9 DRT Spiral Dryers

²1.75 m

FIGURE 41.2 Plate dryer.

DRT is a recent innovation among the nonadiabatic contact dryers. It utilizes heat from a jacketed wall and transmits it to a thin, fast-moving product film rising in a spiral path along the inner wall surface
(Figur e 41.3). A very small qua ntity of the conveying medium is required to move the vapor from the dryer

1. 
   Conveying gas
   
2. 
   Bottom bearing/support unit
   
3. 
   Rotating tube
   
4. 
   Air guide plates
   
5. 
   Moist product
   
6. 
   Product film
   
7. 
   Flow channel
   
8. 
   Heating or cooling jacket
   
9. 
   Dry product and conveying gas
   
10. 
    Head
    
11. 
    Drive for rotating tube
    

⁸ 6 7 ⁴

5

FIGURE 41.3 DRT spiral dryer.
11
10 ⁹
4

3

1
2
DRT is suitable for wat er-wet and solvent-w et chemi cals, polyme rs, flour, and other products that are in a powder y form. For removing low-boiling solvents from polyme ric produ cts, it acts be st as a predryer to a fluid-bed postdry er [9].
Among the advantag es claimed are: (1) increa sed thermal effici ency; (2) low er power consumpt ion; (3) compact ness; (4) gentl e dr ying; (5) reduced produ ct holdup; (6) quick turnaro und; (7) capabil ity of using low-pres sure was te steam for drying; (8) low produ ct moisture content; (9) simple operation; and (10) min-
imum dust explosion potential. The unit also features low gas flow rates, short residence times (3 to 10 s), and high throughput (up to 20 t/h).


41.3.4.10 Miscellaneous Dryers

A number of proprietory dryers suitable for various polymer-drying operations are available in the mar- ket. Among them are the Solidaire, Continuator, Tor- usdisc , and Ther mascrew (Figure 41.4) [10] .

Solidaire is a continuous dryer consisting of a mechanical agitator rotating with a cylindrical hous- ing, usually jacketed for indirect heating. The agitator is equipped with a large number of narrow, flat, adjustable-pitch paddles that sweep close to the

^{since the heat trans fer rate is ve ry high and the c}i^rn^on^se^sr^- surface of the housing. Residence time can be
^{section al gas flow area is smal l.} varied from seconds to 10 min by changing either the
^{The jacketed outer cylinde r rests on a ba se, wh}p^icit^hch of the paddles or the speed of the rotor. High ^{also embo dies the produc t inlet. Product film mo} p^va^ed^sdle speed breaks up agglomerates and continually ^{spirally up the inner wal l, and dry powder is d}ex^isp^-oses new surface to the heat. It has been success-
^{charged at the top.} fully used for drying ABS, PC, polyvinyl alcohol,
^{A steam -heated concentri c displacement body i}p^solyolefins, and other polymers.
^{placed within this cyli nder, which rotates slow ly by an}The Continuator is used primarily for removing ^{extern al-geared motor. M any segme nted air guid}t^ei^sghtly entrapped volatiles and for process applica- ^{are pro vided on the outer surfa ce of the cylin}t^dio^en^rs requiring a long residence time. The mild agita- ^{and are arrange d at a suitab le an gle. The dist} t^aioⁿn^ceemployed in this device provides gentle product

between these plate s and the inner wall must
greater than the product film thickne ss.
m^beixing that minimizes ‘‘short-circuiting’’ while redu-
cing particle breakup. This type of dryer can process

^{Convey ing gas is blown tangent ially by means o}p^f o^alyethylene, PP, PVC, and other polymers.
^{blower into the tube base enteri ng oppos ite a wet-feed}The Torusdisc is another proprietory design par- ^{metering screw. As a resul t, the gas disper ses the} ti^wcu^etlarly useful in processes that require high-capacity ^{feed by intens e mixing. Pro duct film is creat ed by}he^ta^ht^eing or cooling. Its chief advantage is its versatil- ^{inertial force, which threads its way upwar d in a sp}iⁱt^ry^a. ^lA single unit can be varied over a wide range of
^{path until it reaches the exh aust port.} heat transfer coefficients, residence times, and tem-
^{Du ring this flight through the dry er, the con v}p^ee^yr^-ature profiles. It consists of a stationary horizontal ^{ing gas an d the product are heated by the jacke}v^tes^dsel with a tubular rotor on which are mounted the ^{tube wall and the inner displacemen t body surfa} d^ceo^.ubled-walled disks. These hollow disks provide ap- ^{Therefor e, both gas and pro duct tempe rature increa}pr^so^eximately 85% of the total heating surface. It has ^{in the cyli nder with a concomit ant increa se in the}be^ge^an^s used commercially for drying ABS, PCs, poly- ^{absolut e humidi ty. This ensures that product moi}o^sle^-fins, and other polymers.
^{ture is low ered up to the dischar ge poi nt, despite}Thermascrew is a hollow screw, jacketed trough
^{increa sing mois ture pa rtial pr essure. This increa s}dⁱⁿry^ger that provides three to four times more heat ^{driving force gu arante es low er final mois ture conte}tⁿra^t n^ssfer surface than simple jacketed screw conveyors
^{when compared with a co nvection al flash dryer.} and six times more than water-cooled drums. In either

(c) Torusdisc (d) Thermascrew

FIGURE 41.4 Some recent proprietary polymer dryers. (From Bepex Corporation, CEP, 79(4):5 (1983). With permission.)

continuous or batch operation, it provides efficient and uniform heating, cooling, evaporating, or other processing. It can operate in either a pressure or a vacuum environment. Polyester and polyolefins are among the materials dried with good thermal effi- ciency in such devices.
Among recent developments in dryers is the Yamato band FBD [11]. This is a modified FBD having all the components of a standard FBD with an additional carriage means with multiple blades mounted thereon and projecting there from for effective fluidization and transportation of materials (Figure 41.5).
The carriage includes a crank mechanism for effecting a circular or linear movement of the blades. It is driven in such a manner that the blades scratch
and fluidize the material being treated in cooperation with a heated gas. The fluidized bed is thus carried or conveyed toward the outlet port. The blades on the carriage extend in close proximity to the surface of the gas distributor plate. The blades may be straight, curved, or T-shaped. Such dryers can be used to process a variety of difficult-to-treat materials, e.g., slurries and materials containing solidified portions, as well as those having a high degree of cohesion or adhesion and/or containing lumps.
The spouted beds (SBs) can also be used to dry polymer beads. It is an efficient solid–gas contactor. In the conventional SB there is dilute-phase pneu- matic transport of particles entrained by the spouting jet in the central core region and dense-phase down- ward motion of the particles along the annular region

FIGURE 41.5 Yamato band fluidized bed dryer.

bounded by the cylindrical wall. Thus, the particle– gas contact is cocurrent in the core (or spout) and countercurrent in the downcomer or annulus. This characteristic recirculatory motion of the particles enables one to control the residence time of particles within wide limits by letting the particles go through a desired number of cycles prior to their withdrawal. With both batch and continuous operations possible along with the various modifications available, these beds have a strong potential as postdryers in polymer drying [12].

4.