Spurred by continually escalating energy costs, along
with the advent of new competitive polymers accom-
panied by new and extended applications of polymers
and plastics, interest in the energy-intensive operation
of drying of polymers has been on the rise in recent
years.

Drying is one of the prime polymer recovery op-

erations performed before transfer to the compound-
ing plant or packaging for direct use. It is the part of
the process in which the polymer is handled essen-
tially as a solid and liquid and gas streams become
relatively minor. In polymer production, other recov-
ery operations include salvation of the unreacted
monomer and solvent, coagulation and precipitation,
concentration and devolatilization, and liquid–solid
separation [1].

Although drying is the oldest and most commonly

encountered of all unit operations of chemical engin-
eering, it is one of the most complex and least under-
stood operations. One of the prime reasons for this
state of affairs is the enormous diversity of drying
equipment; over 100 clearly identifiable different
types of dryers are in commercial use around the
world. Depending upon the nature of the processing
mode, physical state of the feed, mode of heat and
mass transport, operating temperature and pressure,
and other factors, one can classify existing dryers into
so many different types that it is impossible to de-
velop or hope to develop generalized procedures for
analysis of all types of dryers [2].

This chapter provides a few guidelines for the

selection of polymer dryers and discusses the alterna-
tives available. Picking the best dryer for a specific
polymer application is beyond the scope of this chap-
ter because of many variables involved in such selec-
tion. Since most of the work in this area is proprietory
in nature with very little information available in the
open literature, it is therefore believed that this chap-
ter will help to overcome the initial agonies of a
polymer engineer in selecting dryers for water- or
solvent-wet granular polymer particles.

For better understanding of the subject for a

nonspecialist in polymer technology, a brief survey

is given about the various polymerization techniques
employed commercially before discussion of the dry-
ing equipment.

41.2 COMMON POLYMERIZATION

PROCESSES

The selection of dryers for polymer drying depends to
a large extent on the upstream operations, e.g., poly-
merization, since the behavior of a polymerization
reaction and the properties of the resulting polymer
can vary greatly according to the nature of the physical
system in which the polymerization reaction is carried
out. Several processes are used commercially to pre-
pare polymers. Each process has its advantages, usu-
ally depending on the type and final use of the polymer.
The following types of polymerization processes based
on physical systems are considered briefly in this sec-
tion: (1) bulk; (2) solution; (3) suspension; (4) emul-
sion; and (5) gas-phase polymerization [3].

41.2.1 B

ULK

OLYMERIZATION

The polymerization of the pure monomer without
diluent is called bulk polymerization or mass polymer-
ization. The monomer (e.g., styrene, vinyl chloride
(VC), vinyl acetate (VA), acrylic esters, butadiene,
or acrylonitrile) is first purified to remove oxygen or
other inhibitors (by bubbling nitrogen through it, by
distillation, or by evacuation) and then the polymer-
ization is started through heating, ultraviolet (UV)
radiation, or the addition of an initiator (e.g., perox-
ides, azo compounds, and others). Usually, after a
short period of heating, the reaction mixture con-
tinues to heat by itself, and therefore it is necessary
to remove the heat by cooling. With increasing con-
version, because of the rapidly increasing viscosity of
the polymer–monomer mixture, this becomes more
and more difficult. With large amounts of monomer,
bulk polymerization often takes very turbulent and
even explosive form as a result of the rapidly increas-
ing temperature. The violence of the reaction is even
further increased by the increase in the radical con-
centration that occurs with increasing viscosity.

2006 by Taylor & Francis Group, LLC.

Because of the difficulty in heat removal, bulk poly-

merization is only carried out in a few cases. However,
in the cases in which it is used, it is done on a very large-
scale, e.g., the bulk polymerization of styrene or
ethylene (high-pressure process). Since in this type of
polymerization the possibility of chain transfer is rela-
tively small and because of self-acceleration, other poly-
mers with high molecular weights are found. One of the
characteristics of this process that is always a technical
advantage is the great purity of the polymer resulting
from the lack of additives during polymerization.

41.2.2 S

OLUTION

OLYMERIZATION

For solution polymerization, a solvent inert to the
monomer is used to control the polymerization. High
exothermicity is limited by dilution, causing the reac-
tion rate to be slowed owing to solvent addition. The
solvent is recycled after cooling and is sent back to the
polymerization reactor. The concentration of the solv-
ent is chosen in such a way that the polymerization
mixture can still be stirred after complete conversion.

Solution polymerization has been employed almost

exclusively in cases in which the polymer is then used in
the form of solutions (50 to 60%) for lacquers, adhe-
sives, impregnation materials, and other products.
Obtaining the pure polymer by distilling off the solvent
is complicated because the hard polymer cannot be
taken out of the vessel after evaporation of the solvent.
Through construction of extruders with vacuum distil-
lation zones and by using other special evaporators, it is
possible to separate the polymer from the solvent.

In this process the choice of the solvent’s chain

transfer constant is very important because this influ-
ences the molecular weight to a considerable extent.
Because of chain transfer with the solvent and be-
cause of the lower monomer concentration, the mo-
lecular weight of polymers prepared by solution
polymerization is usually lower than that of the cor-
responding bulk polymers.

Commercially, solution polymerizations are not

carried out to high conversions (near 100%) but
continuously at a constant monomer concentration.
The unreacted and evaporated monomer is recycled
together with the solvent. This type of production
process has two advantages. The reactor always
works in a range of high polymerization rates, and
the molecular weight distribution curve is not so
broad as it is with polymers produced in a discontinu-
ous process with high conversions.

41.2.3 S

USPENSION

OLYMERIZATION

In the suspension polymerization process, water is
used to control heat generation. A catalyst is dis-

solved in the monomer, which is dispersed in water.
A dispersing agent is incorporated to stabilize the
suspension formed. For any nonpolar monomers,
this method offers a method of eliminating many of
the problems encountered in bulk and in solution
polymerization, especially the heat dissipation prob-
lem in the former and solvent reactivity and removal
of the latter.

Another attractive feature of large batch prepar-

ations is that the polymeric products obtained from a
suspension polymerization, if correctly carried out,
are in the form of finely granulated beads that are
easily filtered and dried.

On a technical scale, suspension polymerization is

used in the production of polyvinylchloride, polystyr-
ene, polymethyl methacrylate, and others. For the
production of rubbery, sticky, polymers (e.g., the
polyacrylates), this is less suitable.

41.2.4 E

MULSION

OLYMERIZATION

The emulsion polymerization process is similar to
suspension polymerization. This process is also car-
ried out in a water medium. An emulsifier, either
anionic soap or cationic soap, is added to break the
monomer into very small particles. The initiator is in
solution in the water. After polymerization, the poly-
mer can be precipitated, washed, and dried, or the
mixture can be used directly (e.g., latex paint).

Emulsion polymerization is superficially related

to suspension polymerization, but the kinetic rela-
tionships are entirely different. The major causes of
the differences are: first, the monomer droplets in the
latter system are approximately 0.1 to 1 mm in size
and the particles in the former are approximately
10

to 10

mm in size; and second, the catalyst

is dissolved in the aqueous phase in the latter but is
incorporated directly into the droplets in the former.

In all cases in which the presence of an emulsifier

is not disturbing, emulsion polymerization is advan-
tageous. In comparison with other polymerization
techniques, it has the following advantages: (1) the
polymerization heat can be removed very easily and
(2) the viscosity of the lattices, even with high con-
centration (up to 60%), is low in comparison with
corresponding solutions.

One large-scale use of this process is in the

production of synthetic rubber and, on a small-scale,
in the production of polyvinyl chloride (PVC) and
polystyrene. The other large-scale use is in the pro-
duction of plastics dispersions used as such (without
first coagulating them) for the production of paints,
pigments, inks, coatings, and adhesive paste (e.g.,
polyvinyl acetate, polyvinyl propionate, and poly-
acrylic ester dispersions).

2006 by Taylor & Francis Group, LLC.

41.2.5 G

-P

HASE

OLYMERIZATION

The term gas-phase polymerization is a misnomer in
that it refers only to a polymerization reaction initi-
ated on monomer vapors, generally by photochemical
means. High-molecular weight polymer particles are
not volatile, so a fog of polymer particles containing
growing chains quickly form and the major portion of
the polymerization reaction occurs in the condensed
state.

High-uniformity

polyethylene

(PE)

can

manufactured by passing gaseous ethylene through
an active chromium-containing catalyst bed. Other
monomers that have been polymerized successfully
in the gas phase include methyl methacrylate, VA,
and methyl vinyl ketone [4].

41.3 DRYER CLASSIFICATION [5–7]

41.3.1 C

LASSIFICATION BY

ODE OF

EAT

RANSFER

41.3.1.1 Indirect Dryers

Indirect dryers, also called nonadiabatic units, separ-
ate the heat transfer medium from the product to be
dried by a metal wall. These dryers are subdivided on
the basis of heat applied by radiation or through heat
transfer surface and also by the method in which
volatile vapors are removed.

Heat transfer fluids may be of either the condens-

ing type (e.g., steam and diphenyl fluids, such as
Dowtherm A) or the liquid type (e.g., hot water and
glycol solutions). Because of low film coefficients of
the noncondensing gaseous system, it is seldom used
as the heating medium.

Indirect dryers have several distinctive operating

features: (1) the risk of cross-contamination is
avoided since the product does not contact the heat-
ing medium; (2) since a limited amount of gas is
encountered, solvent recovery is easier than with an
adiabatic dryer; (3) dusting is minimized because of
the small volume of vapors involved in indirect dry-
ing; (4) dryers allow operation under vacuum or in
closely controlled atmospheres that can avoid prod-
uct degradation; and (5) explosion hazards are easier
to control.

Typically, indirect dryers are used for small- or

medium-size production. The product from such a
unit has a higher bulk density than the same material
processed in direct dryers. Particle size degradation
usually can be minimized by proper selection of agi-
tator speed or design. The common indirect heated
dryers are tubular dryers (with or without vacuum),
drum dryers (atmospheric, vacuum, horizontal or ro-
tary vacuum, and others), hollow disk dryers, paddle

dryers, mechanically fluidized bed dryers, pneu-
matically conveyed dyers, cone or twin-shell dryers,
and others.

41.3.1.2 Direct Dryers

Direct dryers or adiabatic or convective dryers trans-
fer heat by direct contact of the product with the hot
gases. The gases transfer sensible heat to provide the
heat of vaporization of the liquid present in the solid.

Direct dryers may use air, inert gas, superheated

vapor, or products of combustion as the heating
medium. Combustion gases are seldom used in poly-
mer drying because of possible product contamin-
ation. Inert gas eliminates the explosion and fire
hazard and may be desirable to prevent oxidation of
polymers prior to the introduction of stabilizers. Use
of superheated vapor as a heat carrier is highly desir-
able when solvent is vaporized in the dryer and has to
be recovered.

Commonly used direct dryers in polymer plants

are rotary warm air, fluidized bed, flash, spray, tun-
nel, and various vibrating and spouted bed (SB)
types. All these have a common disadvantage. The
amount of air or hot gas required is fairly large, which
causes the auxiliary equipment needed (e.g., air heat-
ers, blowers, and dust collectors) to be sized accord-
ingly; the thermal efficiency is also lower than that of
indirect dryers.

Although this classification of dryers has some

importance, it is quite difficult to apply it in more
than a general way. Both types of dryers are commonly
used in polymer-drying processes. Often a combin-
ation of direct and indirect drying is economically
the most efficient solution to some polymer-drying
problems.

41.3.2 C

LASSIFICATION BY

ESIDENCE

IME

The pressing need of product quality in the plastics
industry also forces one to consider residence time
distribution of the product when comparing dryers.

41.3.2.1 Short Residence Time

The short residence time category comprises spray
dryers, pneumatic dryers, and thin-film dryers in
which the residence time may be of the order of
several seconds.

41.3.2.2 Medium Residence Time

Continuous fluid-bed dryers, steam tube rotary
dryers, and rotary dryers can be designed to provide
medium residence time (of the order of minutes).

2006 by Taylor & Francis Group, LLC.

41.3.2.3 Long Residence Time

Rotary dryers, batch fluid dryers, continuous or
batch tray dryers, hopper dryers, multispouted bed,
and vacuum tumble dryers are typical long-residence
units used in polymer drying.

41.3.3 O

THER

ONSIDERATIONS

On the basis of the polymerization alone, it is difficult
to specify definite dryer selection rules since typically
polymer properties differ over a wide range. The choice
of dryer is also limited by the physical properties of the
polymers, e.g., polymer-handling characteristics, indi-
vidual or closely related drying curves, properties of the
emitted volatiles, limitations on temperature, and par-
ticle size and distribution requirements. Other factors
include equipment space limitations, production rates,
pollution control requirements, solvent recovery,
thermal sensitivity, and product quality specifications.

The primary step in specifying a dryer is to define

the physical, thermal, and chemical properties of the
product and the volatiles present. Often the consist-
ency of the feed reduces the choice of dryer. A few
guidelines are always helpful in selecting polymer
dryers. For example, if a solvent must be evaporated
and then recovered, it is usually not desirable to choose
a convection dryer. Since solvent must be condensed
from a large carrier gas flow, the condenser and other
equipment become rather large. If the maximum prod-
uct temperature is lower than about 308C, it is possible
to specify a vacuum dryer. If the average particle size is
about 0.1 mm or larger, a fluidized bed dryer may be
considered, or if the feed is a slurry or paste a spray
dryer may be a judicious choice. Scaling is another
important factor that can dictate dryer selection. For
example, if the requirements are to produce high ton-
nage of a polymer in one line, it probably would be
advantageous to consider a fluid-bed dryer rather than
a mechanical rotating type.

The reader is referred to other chapters of this

handbook for details concerning specific dryers
discussed.

41.3.4 C

OMMON

OLYMER

RYERS

41.3.4.1 Rotary Dryers

Historically, rotary dryers (RDs) have been the most
popular in polymer-drying operations. A rotary dryer
consists of a slowly revolving drum (often fitted with
internal flights or lifters) through which both the
material and the gas pass. Gas, cocurrent or counter-
current with the granular polymer, can be introduced
at either end of the cylindrical shell.

Solids move through the dryer by the effect of grav-

ity, the rotation of the cylinder, and gas flow (in the case
of cocurrent units). Internal scoops, blades, and lifters,
which give the solids a showering pattern, are provided
for better gas–solid contact. Baffles and dam rings are
also available to retard the forward motion of the solids
and to increase residence time (5 to 20 min is common;
much larger times are also found in drying of certain
polymer pellets).

An improvement over the standard rotary dryer is

the steam tube rotary dryer. Here, two or three rows
of steam tubes are located in concentric circles within
the shell, which extend the full length of the cylinder.
The tubes together with a series of small radial flights
serve to agitate the material for uniform drying.
These types of dryers were used in the polymer indus-
tries for heat-sensitive polymers requiring indirect
heating.

With the advent of the new and energy-efficient

dryers, rotary dryers nowadays are seldom used in
polymer drying in new polymer plants. Modified
fluid-bed dryers as well as novel spouted bed dryers
can replace rotary dryers in many applications.

41.3.4.2 Flash Dryers

The flash dryer (FD) is a direct-type, cocurrent unit
that is essentially a long vertical tube with no moving
parts. In polymer drying this is mostly used as a
predryer to remove surface moisture.

In FD units, hot inlet gases contact the wet prod-

uct, which may be powdery, granular, crystalline, or
pasty material, as discharged from a centrifuge or
filter press. Providing a short residence time of several
seconds, FD is well suited for high evaporative loads.
Drying is nearly adiabatic, an advantage with heat-
sensitive polymers. High mass and heat transfer rates
are obtained because of the high relative velocity
between feed and inlet gas and a large exposed prod-
uct surface area.

The method of feeding wet polymer to FD is very

important. Granular products are relatively free flow-
ing when wet polymers are fed with devices such
as screw and rotary star feeders; sticky polymers
may be best handled with a table feeder. Lumpy or
pasty polymer must be broken up or mixed with dry
product recycle to produce a more uniform and
free-flowing feedstock.

Among the developments in flash drying, the first

and the simplest is the ‘‘thermo venturi’’ drying con-
cept in which a vertical drying column expands so
that coarse particles remain suspended while drying
and finer particles travel straight through with the
drying air. This is quite effective as long as the par-
ticles are relatively spherical and the size spread is not

2006 by Taylor & Francis Group, LLC.

too great. Similar designs feature ‘‘bicones’’ in which
the drying column expands and contracts, possibly
with the addition of supplementary hot air, often
injected tangentially.

Recent improvements in flash drying include the

ring dryer. The heart of this dryer is a centrifugal
separator. It combines renewal of the drying air with
centrifugal classification. The lightest and finest frac-
tions of the product are passed with the spent drying
medium into the product collection system; oversize,
partially dried material is held in circulation. The split
is varied by adjusting the positions of suitable deflec-
tors, introduced in the flow loop. This type of FD is
available in both multistage and closed-circuit designs
with both direct and indirect heating options for re-
moving both surface and bound moistures, as well as
solvent removal and recovery.

41.3.4.3 Spray Dryers

In spray dryers, the feed material, in the form of a
solution, suspension, slurry, or paste, is sprayed in a
high-temperature gas zone by centrifugal disks or
pressure nozzles. Such dryers are used in polymer
industries in which the polymers cannot be separated
mechanically from the carrier liquid, e.g., emulsion-
polymerized PVC.

In polymer industries, wherever spray dryers are

used they are primarily used as predryers of a multi-
stage system. Final drying is normally done in a fluid
bed, which is either stationary or vibrated type. Sta-
tionary fluid beds are used when spray-dried powder
leaving the drying chamber is directly fluidizable. The
vibrated type of fluid bed is used for products that, on
leaving the spray dryer, are not readily in a fluidizable
state owing to their particle form, size distribution, or
wetness.

In such a multistage system, the higher moisture

content powder leaving the spray-drying chamber is
transferred to the second stage, which is a fluid bed
for completion of drying. The higher inlet temp-
erature and lower outlet temperature operation in
such a system give improved dryer thermal efficiency
and increased dryer capacity without product quality
degradation.

41.3.4.4 Fluidized Bed Dryers

Fluidized bed dryers (FBDs) involve the suspension
of solid particles in an upwardly moving stream of
gas, which is introduced through a distribution plate
that may be cooled for heat-sensitive polymers. Such
a dryer may operate batchwise.

The advantages offered by FBDs are: (1) the

even flow of fluidized particles permits continuous,

automatically controlled, large-scale operation with
easy handling of feed and product; (2) no mechanical
moving parts, i.e., low maintenance; (3) high heat
and mass transfer rates between gas and particles—
this is well mixed, which also avoids overheating of
the particles; (4) heat transfer rates between fluidized
bed and immersed objects, e.g., heating panels, are
high; and (5) mixing of solids is rapid and causes
nearly isothermal conditions throughout the bed,
thereby facilitating easy and reliable control of the
drying process.

Using the solvent being removed as the heat car-

rier and fluidizing medium (i.e., a superheated vapor)
has proved a feasible and beneficial design. Its advan-
tages include: (1) reduction in size of condensing and
recovery equipment; (2) increase in drying rate due to
the elimination of the gas-film resistance of the for-
eign vapor; (3) volumetric heat capacity of various
vapors is usually greater than that of air; and (4)
space velocity for fluidization is lower than with air,
which reduces the volumetric vapor flow and conse-
quently the size of the dust collector, air moving
equipment, and other parts.

Drying of polystyrene beads is a typical example

for industrial use of these dryers because of the close
range of bead particle size. Also, the size of the beads
permits high fluidizing velocities and therefore eco-
nomic dryer sizes.

In recent years, indirect-heated fluidized beds have

made inroads in almost all industries. Some of their
advantages over the direct-heated FBD are: (1) the
indirect heat transfer rate significantly reduces gas
flow requirements; (2) there is tremendous leverage
gained by the multiple of the heat transfer coefficient,
LMTD, and heat transfer surface density permits
very high heat inputs into low-temperature, heat-
sensitive applications; (3) when a plug-flow, rectan-
gular indirect fluid bed or low bed height is used, the
solids flow counter to the thermal fluid, behaving like
a countercurrent heat exchanger with all its attendant
benefits; and (4) since the heat source is decoupled
from the fluidizing gas source, vessel diameters and
pollution-control equipment are much smaller.

Indirect fluid beds have already proved efficient

in drying very heat-sensitive polymers with large
constant-rate drying periods, as in drying PVC,
polyethylene, acrylonitrile–butadiene–styrene (ABS)
copolymers, and polycarbonates (PC).

41.3.4.5 Vibrated Fluidized Beds

A vibrated fluidized bed (VFB) is basically a long
rectangular trough vibrated at a frequency of 5 to
25 Hz with a half amplitude of a few millimeters
(2 to 5 mm). This kind of dryer can be used for drying

2006 by Taylor & Francis Group, LLC.

wet, sticky , an d granula r media an d has be en used
success fully for drying pol ymers. It is often use d as a
second-s tage dryer afte r a flash or spray dryer in
many polyme r-drying applic ations .

Bene fits achieve d from such dryers are: (1)

unifor m resid ence tim e distribut ion regardless of
particle size; (2) a bility to han dle pol ydisperse soli ds;
(3) ability to operate at low aeration rates and hence
lower pressure drops; (4) gen tle ha ndling of pro duct;
(5) higher heat trans fer and drying rates, and others .
Since the equipment is mo unted on resonan ce spring s,
the power co nsumpt ion for vibrat ion is mini mal for
well-d esigne d VFB [6].

The vibration vector is typicall y a pplied at a smal l

angle to the vertical to permit conveying of the solid s
in the long direction at the desir ed rate. This permi ts
control of resi dence times an d also be tter con trol of
the drying or heati ng rates as the material progres ses
downst ream.

41.3.4 .6 Cont act Flui d-Bed Dryers

Conta ct FB units are ch aracterize d by the residence
time distribut ion of the individ ual pa rticles insi de the
unit. A broad resi dence time dist ribution is obtaine d
in a back -mixed FB in which the lengt h/width rati o of
the be d is relat ively small. The na rrow residen ce time
distribut ion is obtaine d in a plug-flow FB in which
the lengt h/widt h ratio of FB is very large . This cor-
responds to a long, narrow FB. Alternat ively, this can
be obt ained by compart menta lizing FB and is the
usual practice followe d in the indust ry.

Compared with the plug-flow FB, a back-mixed FB

has a significant advantage inasmuch as the back-mixed
FB can accept a feed material that is not readily fluidiz-
able. This is possible owing to the vigorous mixing
inside FB and that the material inside the bed acts as a
large reservoir in which incoming feed material will be
dispersed and the surface moisture will be flashed off,
making the product fluidizable. This characteristic
makes the back-mixed FB concept well suited as the
predrying stage in many polymer-drying systems.

The plug-flow FB drying con cept is parti cularly

suitable for drying bound mois ture from heat-sensi tive
material s since the resi dence time is control led wi thin
narrow lim its. In the typical polyme r applic ation, this
means that the bound mois ture can be remove d from
the polyme r product at the low est possible produ ct
tempe rature.

Thes e two concepts, along with heating of the bed

indirec tly by imm ersed heat exchange surfa ces, are
jointly util ized in co ntact FBD. In FB applic ations
for polymers with indirect heating, the temperature of
the heating panels is typically limited by the softening
point of the polymer.

41.3.4.7 Paddle Dryers

The paddle type of dryer, marketed by Nara Machin-
ery Company of Japan, is an indirect dryer for granu-
lar or powdery material that dries such materials by
bringing them into contact with revolving, cuneiform
hollow heaters (paddles) without using gas as a heat-
ing medium. The paddles revolve at a low speed (10 to
40 rpm) inside the grooved trough fitted with a jacket
(

Figur e 41.1

) .

The heating medium passes inside the hollow pad-

dle so that the entire surface of the paddles and shafts
acts as the heat transfer surface. The cuneiform blade
enhances agitation of the material and at the same
time prevents the powder from adhering to the heat
transfer surface. For greatest heating efficiency the
dryer is tilted slightly in the direction of product
flow and is designed so that the material contacts all
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
out by a flow of hot air or other gas.

The main features of the paddle dryer are: (1) it is

compact; (2) has high heat transfer coefficient and
good thermal efficiency; (3) the paddles have an inter-
play for self-cleaning; (4) it is easy to control; and
(5) a small amount of gas is required that minimizes
dusting and other problems.

Paddle dryers have been successfully used in dry-

ing such polymers as VC resin, nylon pellets, and
polypropylene (PP), as well as polyethylene. Operated
in a closed-cycle mode they can recover organics from
such solvent-laden products as polyethylene or PP
and can reduce the air volume requirement to only
5 to 10% of that used in direct dryers.

Energy requirements for such dryers are also

lower. It is seen that 1300 to 1500 Btu is required to
dry 1 lb of moisture with the paddle dryer compared
with 3000 Btu/lb for a suspended air unit. Because of
the smaller air volume needed, the sizes of down-
stream condensers and refrigeration system units are
reduced.

41.3.4.8 Plate Dryer

The plate dryer (PD) is an indirect dryer in which heat
transfer is accomplished by conduction between the
heated plate surface and the product. It comes under
three major variations, e.g., atmospheric, gas tight,
and vacuum.

In these dryers, the product to be dried is metered

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

2006 by Taylor & Francis Group, LLC.

across stationar y plate s. The plates are heated by a
liquid medium or steam. Small plate s with intern al
rims an d large plate s with exter nal rim s are arrange d
in an alte rnating sequ ence (Figure 41.2) .

Thi s arrange ment makes the product drop from

the outsi de edge of the small plate down to the large
plate, on which it is co nveyed to the insid e ed ge, and
then drops to the foll owing smaller plate, where it is
conveyed again toward the ex ternal edge. This de sign
of the conveying system en sures plug flow of the
produc t throu ghout the entir e dryer. Each plate or

group of plate s may be heated or cooled indivi dually ,
thus offer ing precis e con trol of the prod uct tempe ra-
ture an d the possibili ty of adjust ing a tempe rature
profile dur ing the drying process . Thermal deg rad-
ation of sensitiv e mate rials can thus be avoided, and
cooling subsequent to drying can be achieve d.

In the plate dryer, the produ ct layer is kept shal-

low (approxi mate ly 10 mm). The entir e plate surface
is utilized for heat trans fer. The product su rface ex-
posed to the surround ing atmos phere is even large r
than 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
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
heat exc hange surface betw een 3.8 and 175 m

41.3.4.9 DRT Spiral Dryers

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

FIGURE 41.1 Paddle dryer.

1. Product

2. Heating or

cooling
medium

3. Shell

housing

4. Conveying

system

5. Plate

FIGURE 41.2 Plate dryer.

2006 by Taylor & Francis Group, LLC.

since the heat trans fer rate is ve ry high and the cross-
section al gas flow area is smal l.

The jacketed outer cylinde r rests on a ba se, which

also embo dies the produc t inlet. Product film mo ves
spirally up the inner wal l, and dry powder is dis-
charged at the top.

A steam -heated concentri c displacement body is

placed within this cyli nder, which rotates slow ly by an
extern al-geared motor. M any segme nted air guides
are pro vided on the outer surfa ce of the cylind er
and are arrange d at a suitab le an gle. The dist ance
between these plate s and the inner wall must be
greater than the product film thickne ss.

Convey ing gas is blown tangent ially by means of a

blower into the tube base enteri ng oppos ite a wet-feed
metering screw. As a resul t, the gas disper ses the wet
feed by intens e mixing. Pro duct film is creat ed by the
inertial force, which threads its way upwar d in a spira l
path until it reaches the exh aust port.

Du ring this flight through the dry er, the con vey-

ing gas an d the product are heated by the jacketed
tube wall and the inner displacemen t body surfa ce.
Therefor e, both gas and pro duct tempe rature increa se
in the cyli nder with a concomit ant increa se in the gas
absolut e humidi ty. This ensures that product mois -
ture is low ered up to the dischar ge poi nt, despite
increa sing mois ture pa rtial pr essure. This increa sing
driving force gu arante es low er final mois ture content s
when compared with a co nvection al flash dryer.

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
inner surface of the housing. Residence time can be
varied from seconds to 10 min by changing either the
pitch of the paddles or the speed of the rotor. High
paddle speed breaks up agglomerates and continually
exposes new surface to the heat. It has been success-
fully used for drying ABS, PC, polyvinyl alcohol,
polyolefins, and other polymers.

The Continuator is used primarily for removing

tightly entrapped volatiles and for process applica-
tions requiring a long residence time. The mild agita-
tion employed in this device provides gentle product
mixing that minimizes ‘‘short-circuiting’’ while redu-
cing particle breakup. This type of dryer can process
polyethylene, PP, PVC, and other polymers.

The Torusdisc is another proprietory design par-

ticularly useful in processes that require high-capacity
heating or cooling. Its chief advantage is its versatil-
ity. A single unit can be varied over a wide range of
heat transfer coefficients, residence times, and tem-
perature profiles. It consists of a stationary horizontal
vessel with a tubular rotor on which are mounted the
doubled-walled disks. These hollow disks provide ap-
proximately 85% of the total heating surface. It has
been used commercially for drying ABS, PCs, poly-
olefins, and other polymers.

Thermascrew is a hollow screw, jacketed trough

dryer that provides three to four times more heat
transfer surface than simple jacketed screw conveyors
and six times more than water-cooled drums. In either

8 6 7 4

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

FIGURE 41.3 DRT spiral dryer.

2006 by Taylor & Francis Group, LLC.

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

(a) Solidaire

(b) Continuator

(d) Thermascrew

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

IIc

FIGURE 41.5 Yamato band fluidized bed dryer.

2006 by Taylor & Francis Group, LLC.

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].

41.4 TYPICAL DRYING SYSTEMS

FOR SELECTED POLYMERS

This section discusses briefly the drying of selected
large-scale polymers. It is important to note the data
in Table 41.1, which gives permissible moisture levels
in various commodity resins [13].

41.4.1 D

RYING OF

OLYOLEFINS

41.4.1.1 Polypropylene

PP is produced by a variety of processes, most of them
by a diluent phase propylene polymerization utilizing
a Ziegler–Natta-activated titanium trichloride catalyst
in the presence of low- to high-boiling hydrocarbons.
Residual catalyst removal followed by hydrocarbon
slurry centrifugation is the immediate upstream oper-
ation prior to thermal drying. Hexane is the solvent
used in the major PP processes in operation today. As
a result these polymers are solvent wet.

Many plants operate with two resin varieties,

e.g., homopolymers and copolymers. Each requires

a different drying approach owing to different centri-
fuge cake-handling characteristics. Homopolymer
cakes, although somewhat tacky, are much less than
high-ethylene-content copolymer cakes, which tend to
agglomerate, form lumps, adhere to surfaces, and so
on. Considering capital cost, it is desirable to have a
single dryer line for both resins. Consequently, initial
dryer selection becomes a critical issue because of the
feed flexibility required [14].

Both polymer centrifuge cakes are discharged hot

(50 to 608C), with diluent contents as high as 35 (wb)
or 53.2% (db). Between 35 (wb) and 5% (wb), most
homopolymers and copolymers exhibit constant-rate
drying characteristics; i.e., all moisture evaporation is
from the particle surface. Drying is rapid, and resi-
dence time is heat transfer-dependent. Since the prod-
uct temperature limit for these polymers is 100 to
1108C, the solvent boiling point has a definite effect
on dryer selection. Historically, again, rotary dryers
are used. During the 1960s and early 1970s, a two-
stage system of paddle-type dryers was used success-
fully. This consisted of a first stage or surface solvent
dryer, with the characteristics of very high agitation,
high heat transfer, and short residence time. The sec-
ond stage, or bound moisture dryer, consisted of a
device with low agitation, low heat transfer, and long
resistance. Each dryer is provided with a recycle purge
gas system to aid in controlling dew points and in-
crease dryer efficiency. The gas flow is minimized; the
amount used is that required to give a partial pressure
necessary to achieve required product moistures.

With the emergence of very high capacity PP

polymer lines and high-boiling point solvents, various
types of dryers and drying systems have evolved. One
reason for the advent of the new technologies in PP

TABLE 41.1
Percentage by Weight of Permissible Moisture (db) in Some Selected Polymer Resins

Material

Permissible Moisture

Drying Temperature (8C)

Injection (%)

Extrusion (%)

ABS resin

0.10–0.20

0.03–0.05

77–88

Acrylic

0.02–0.10

0.02–0.04

71–82

Cellulosics

Max. 40

Max. 30

66–88

Ethyl cellulose

0.10

0.04

77–88

Nylon

0.04–0.08

0.02–0.06

Polycarbonate

Max. 0.02

0.02

121

Polyethylene

—

Low density

0.05–0.10

0.03–0.05

71–79

High density

0.05–0.10

0.03–0.05

71–104

Polypropylene

0.05

0.03–0.10

71–93

Polystyrene

0.10

0.04

71–82

Vinyl

0.08

60–88

2006 by Taylor & Francis Group, LLC.

drying is the economic recovery of the flammable
hydrocarbon solvents. Another reason is that PP
has to be dried to a very low volatiles level and
the final drying requires the drying gas to have an
extremely low dew point. Usually nitrogen gas is
used as a drying gas in a closed-cycle drying system.
Figure 41.6 shows this by low and high gas dew point
product moisture points indicating relative drying
times. Reduction in recycle gas dew point is required
to remove evaporated solvent first and, especially in
mass transfer limited drying, reduce solvent partial
pressure to increase the overall drying rate.

Accordingly, the low dew point case normally uses

208C hexane dew point recycle nitrogen gas yielding

the lowest residence time but at a higher energy ex-
pense than the higher dew point case. Since this is an
expensive part of the flow sheet, refrigeration costs
become a factor and recycled gas should be minim-
ized. It is in this region that residence times vary from
30 min to over 1 h, depending on recycle gas dew
point and polymer-drying characteristics; conse-
quently, a controlled residence time dryer is desired.
It is also desirable that this postdryer has an inde-
pendent recycling loop to minimize energy consump-
tion and maximize process control.

Based on these fundamentals, a two-stage flash or

fluid-bed drying system has been developed for PP
drying. The flash dryer disperses the feed cake in a
venturi throat, with hot recycled gas breaking the
cake and drying it to about 5% (wb) level. Final
drying is carried out in the fluid bed in a nitrogen
atmosphere. The solvent is recovered by a scrubber–
condenser system. The PP, after being dried in the
fluid bed, contains an extremely low level of solvent
(e.g., hexane or heptane), typically 500 ppm.

A very recent development in terms of heat econ-

omy and corrosion control is the use of a spiral DRT
dryer (Drallrohr Trocking) as a predryer in place of
the flash predryer [9] in flash–plug-flow FBD systems.
The gas/solid ratio is approximately 0.2 in these types
of dryers, compared with 1.0 in the flash dryers.

Another important advantage of DRT dryer as a

predryer in PP drying is its suitability in a corrosive
environment. A persistent problem often seen in PP
and high-density polyethylene (HDPE) manufactur-
ing plants is the deterioration of the equipment due to
free chlorides. The chlorides result from the deactiva-
tion of the activated catalysts with alcohol. Stress
corrosion cracking is the most common corrosion
phenomenon that results from the catalyst’s chloride

Post fluid-bed dryer, plug flow with gas

Dew point control

DRT/backmix

Minimum fluidization
homopolymer

Minimum fluidization
copolymer

Product diluent

Low dew point

drying gas

High dew point

drying gas

Drying rate is heat transfer controlled

Drying rate is mass transfer controlled

Time

Break point:
constant rate/falling rate

Flash

0.35

0.30

0.25

0.20

0.15

0.10

0.05

0.02

0.0005

(0.532)

Dry basis

Fraction hexane. wet basis

FIGURE 41.6 Typical polymer-drying curve of polypropylene.

2006 by Taylor & Francis Group, LLC.

remnants. The corrosion rate becomes remarkable
when the product contains a very small amount of
water. To prevent such corrosion, a neutralization
liquid is condensed. Also, part of the equipment is
sometimes coated with an acid-proof resin. Despite
the best neutralization techniques, chlorides are al-
ways present and cause significant equipment deteri-
oration unless precautionary measures are taken prior
to the drying system design. Therefore, it is essential
that the constant-rate drying period be run in an
atmosphere that precludes potential hexane vapor
condensation. DRT has advantages in such a corro-
sive environment since it operates with low gross heat
input and product inventory [15].

41.4.1.2 High-Density Polyethylene

HDPE is usually presented to the drying system from
a decanter centrifuge, either water wet or wet with
solvent (e.g., hexane or heptane). The product tem-
perature limit for this polymer is in the range of 1008
to 1108C. This influences dryer selection. Similar to
PP drying, HDPE drying technology progressed
along the same route because of similarity in the
upstream physical operations prior to drying, as well
as similar physical characteristics. Similar to PP, dry-
ing of HDPE is best done in a multistage system,
especially on FD/CFBD centrifugal FBD system
(Figure 41.7).

41.4.2 D

RYING OF

OLYVINYL

HLORIDE

41.4.2.1 Emulsion Polyvinyl Chloride

Historically, spray dryers were used because of their
ability to produce a constant quality product
under full operational control. Normally, emulsion
PVC (E-PVC) is water wet in a slurry and dried to a
powder in one single-pass operation with high cap-
acities. The slurry is atomized using a rotary wheel or
nozzle. Evaporation takes place under constant and
falling rate conditions. Rapid evaporation maintains
a low temperature of the spray droplets so that high
dry gas temperature can be applied without affecting
polymer quality. Conical spray-dryer chambers are
commonly employed.

An improvement over the conventional open-

cycle adiabatic spray dyers for E-PVC is the recycle
exhaust spray dryer. In this type, up to 50% of the
exhaust stream is recycled to preheat the supply air
makeup from the atmosphere.

An improvement with respect to thermal effi-

ciency is the two-stage dryer. This involves operating
a spray dryer with a fluid-bed afterdryer. By adopting
a two-stage layout with a fluid bed, powder is taken
out of the spray dryer at a lower outlet temperature
with higher moisture content. The cooler but higher
moisture content powder is transferred to the fluid-
ized bed, where the drying is completed to the desired

Feed inlet

Continuator

Heater

Product discharge

Heater

Solvent
cooling system

Fan

Solvent
cooling system

Wet condenser

Solidaire

Wet condenser

FIGURE 41.7 Drying system for polypropylene and polyethylene.

2006 by Taylor & Francis Group, LLC.

extent by controlling the residence time. The overall
heat consumption of the two-stage process is reported
to be about 20% lower than the corresponding single-
stage dryer.

A recent improvement over the above-mentioned

two-stage drying system for drying E-PVC is a spray
dryer with an integrated fluid bed. The basic concept
in this type of dryer is to avoid contact of the wet
powder with any metal surface in the primary drying
stage by transferring wet powder directly into a fluid-
ized powder layer (second drying stage). To achieve
this requirement, the fluid bed is integrated at the base
of the spray-drying chamber.

Another improvement in the design of dryers

for drying E-PVC and polyethylene is a dispersion
dryer that operates on what is known as the jet-drying
principle

and

offered

Fluid

Engineer-

ing International (London) under the name Jet-O-
Dryers. It is a pneumatic dryer of toroidal design
developed from jet-milling principles. It has no moving
parts. It is claimed to offer the following advantages
over conventional flash drying: (1) much shorter dry-
ing times and (2) combination of drying and fine grind-
ing in a single operation to deagglomerate the
materials.

41.4.2.2 Suspension Polyvinyl Chloride

Suspension-grade PVC (S-PVC) and its copolymers
have many possible drying options. Since polymeriza-
tion of this polymer is done by using water as the
dispersion liquid, water and some monomers are pre-
sent in the wet cake. Usually, wet cake with 20 to 25%
water (wb) is obtained after centrifuging slurries. Most
of the water contained in the centrifuge cakes is typic-
ally free moisture, with only a minor part bound mois-
ture. Moreover, the bound moisture in typical S-PVC
is held relatively loosely and is fairly easy to dry off.
Traditionally, a rotary dryer system was applied to
achieve a final moisture content of 0.2%. Rotary dryers
for the purpose are typically 1 to 2 m diameter and
15 to 30 m long, rotating at 4 to 8 rpm. Centrifuged
S-PVC is introduced at the upper and cocurrent with
the hot gas flow. Gas flow contact is enhanced by the
use of longitudinal lifting flights attached inside
the drum wall, the purpose of which is to shower the
material through the hot gas stream.

Recently, a two-stage flash fluid-bed system has

appeared in the market that is preferable to a rotary
drying system (Figure 41.8). Most of the surface
water is removed in the flash dryer stage within

Heater

Blower

Air filter

Fluid bed

Cyclone

Flash dryer

Feed

FIGURE 41.8 Flash fluid-bed dryer for suspension-grade polyvinyl chloride.

2006 by Taylor & Francis Group, LLC.

seconds; some surface and all of the bound moisture
are removed in the fluid-bed stage by holding the
product at suitable drying temperatures for about 30
min.

Usually, wet cake with 22 to 25% water (wb) is fed

to the dryer by a screw conveyor and enters by a
special mill that deagglomerates the feed material,
disperses it into the drying airstream, and accelerates
it to duct velocity. The mill should handle the feed
gently, as PVC is sensitive to high shear.

The flash dryer stage discharges the product to the

fluid bed between 2 and 8% water, with the intermedi-
ate moisture chosen according to the basis of opti-
mization used. Flash dryer air temperature may be
typically 1808C at the inlet and 608C at the outlet,
depending on moisture content and the drying char-
acteristics of the particular resin. As already men-
tioned, S-PVC is sensitive to shear; for this reason,
dry duct velocities are kept low (around 15 m/s) and
care is exercised in handling the dried product.

It is possible to arrive at the required product final

moisture content by flash drying alone but, because of
the residence time available in the flash dryer, the high
temperatures required give an unsatisfactory product.
Moreover, there is a very wide range of S-PVC homo-
polymers, varying in molecular weight, particle size,
and other properties, and all have different dewater-
ing and drying characteristics.

The benefits achieved in a two-stage system are its

ability to handle upsets in inlet moisture in the flash
dryer, a lower energy cost, and a relatively simple
scale-up.

Modest improvements with respect to the most

economical drying of S-PVC are a continuous, sin-
gle-stage, contact fluidized bed dryer, as shown in
Figure 41.9. In this type of dryer, the concepts of
back-mixed fluidization and plug-flow fluidization
are advantageously combined in a single unit. A
broad residence time distribution is obtained in a
back-mixed fluid bed in which the bed itself has a
relatively small length/width ratio. In performance it
can be compared with an agitated tank provided
with overflow, inasmuch as the vigorous mixing in-
side the fluid bed will result in a uniform temperature
and constant average moisture content of the par-
ticles throughout the entire bed. The product dis-
charged from this back-mixed fluid bed has the
same temperature and moisture content as the bulk
material inside the fluid bed. Further, because of the
excellent heat and mass transfer between the fluid-
ized particles and the drying air, equilibrium is
reached between the exhaust air and the product
inside the bed. This type of fluid-bed drying concept
is found to be very suitable for drying surface mois-
ture when residence time has no impact on the dry-
ing performance.

After the mixed-bed section, a plug-flow section is

provided in which the final drying of PVC takes place.
This section is fairly small compared with the back-
mixed section and is usually obtained by dividing the
fluid bed into compartments. This concept is particu-
larly advantageous for drying bound moisture from
heat-sensitive materials since the residence time is
controlled within the narrow limits and a distinct

Contact fluid bed
(back-mixed part)

Plug

flow

Cyclone

Heater

Air filter

Product

FIGURE 41.9 Contact fluidizer for suspension-grade polyvinyl chloride.

2006 by Taylor & Francis Group, LLC.

moisture profile can be obtained along the length of
the unit because of a very low degree of back-mixing.

In this type of drying system for S-PVC, wet PVC

cake is usually transported from the decanter centri-
fuge by a screw feeder to the product distributor of the
back-mixed fluid-bed section. It then flows through an
overflow weir into the plug-flow section where the final
drying takes place. Finally, the product is discharged
through the discharge weir arrangement.

The back-mixed section of the unit is provided

with heating panels; no heating panels are provided
in the plug-flow section, partly because the cost can-
not be justified and partly because of the tendency for
electrostatic deposits on the heating panel encoun-
tered with PVC at low moisture content to decrease
the heat transfer coefficient.

The contact fluidized bed provided with heating

panels appears to have proven to be superior to the
flash fluid-bed drying system from the point of view
of heat economy and overall savings. The contact
fluidizer does have a few limitations. First, it is man-
datory that the polymer material be readily fluidizable
at a moisture level well above the moisture level in the
back-mixed section to avoid defluidization of the bed
during upset conditions. Second, the centrifuge cake
should not be too sticky and have too much tendency
to form agglomerates of the individual polymer par-
ticles. In such a case, a flash dryer is better suited as
the predrying stage as better disintegration takes
place in the venturi section of a flash dryer than in a
back-mixed fluid bed.

Although a fluid bed as a second-stage dryer gives

accurate product temperature control while providing
adequate residence time, depending on the predryer
load, evaporative load in this stage may be small.
This results in a low airflow requirement and makes
fluidization more difficult. In such cases, a vibrat-
ing fluid-bed design is a better alternative. Here,
PVC is conveyed by vibration, permitting varying
gas speeds without affecting the conveying rate or
residence time. Also, with the low airflow rates of
the vibrating fluid bed, the fines pickup problem (nor-
mally associated with high gas flow rates) is minim-
ized and, as the vibration is at a low frequency, the
overall effect of the gas and vibration is to transport
the product gently, minimizing damage. The vibrat-
ing FBD must be among the most important but
underutilized dryer of all granular products.

During fluidization of PVC, electrostatic charges

arise of such magnitude that they affect the hydro-
dynamics of the system. This is disadvantageous for
transfer processes in the bed, e.g., for heat transfer
between the heating surface and the bed. This is a
difficult problem in a fluidized bed because of inten-
sive movement of particles and frequent interparticle

and particle–wall contact. Although charge gener-
ation cannot be prevented, one can limit its magni-
tude (and try to increase its dissipation) by changing
process conditions. One method is the addition of a
small portion of fines to the bulk; this results in the
splitting of agglomerates and disappearance of the
particulate layer at the walls. As a result, the bed
regains its original parameters, which assure intensive
running of processes in the bed.

41.4.2.3 Vinyl Chloride–Vinyl Acetate Copolymer

There is a wide difference in the difficulty of drying
vinyl chloride–vinyl acetate (VC–VA) copolymers
according to the degree of VA content in polymer
and extent of polymerization. If the heat-resisting
property of polymer is too low to use the hot air at
a high temperature (even if the hydroextracting de-
gree in the former stage is generally good, e.g., 13 to
17% wb), then it is difficult to remove VA monomer.
As a result, the necessary retention time becomes
longer compared with that of PVC-homo.

The equipment recommended for this application

is single-stage batch fluidized bed dryer (B-FBD) or a
flash B-FBD system. For proper selection it is neces-
sary to make a detailed study on the basis of specified
conditions.

An important factor that should be taken into

account while drying PVC is the corrosion of the
equipment due to the monomer chloride. Monomer
chloride, which is always present in the wet cake,
induces pitting corrosion and stress corrosion crack-
ing in parts where the powdery materials are pro-
cessed. Those parts, therefore, are made of AISI-
316L and are partially coated with an acid-resistant
coating. It is indispensable to make periodic inspec-
tion of the corrosion condition and to make timely
replacement of the necessary spare parts. Preventive
maintenance is imperative to successful operation.

Another important consideration in drying PVC

is the emission of VC. U.S. EPA emission limitations
of <5 ppm on VC must be strictly maintained. This
criterion on VC sometimes dictates the selection of
the drying equipment for PVC. In other countries the
discharge limit on VC emission may be less stringent.

41.4.3 D

RYING OF

CRYLONITRILE

–

UTADIENE

–S

TYRENE

In general, emulsion processes are used to make ABS
of higher impact strength and bulk or suspension
processes are preferred for materials with less impact
strength. This three-monomer system can be tailored
to end-product needs by varying the ratios in which
they are combined. Acrylonitrile contributes heat

2006 by Taylor & Francis Group, LLC.

stability and chemical aging resistance; butadiene im-
parts low-temperature property retention, toughness,
and impact strength; and styrene adds luster (gloss)
rigidity, and processing ease.

The drying characteristics of ABS polymers

change with changes in composition. Generally, a
centrifuge cake containing 50% moisture (wb) must
be dried to a final product containing less than 0.1%
moisture. The critical moisture composition is around
5%. The allowable product temperature is approxi-
mately 1008C. ABS plastics are mildly hygroscopic; if
dried, ABS is left in storage for some time and it must
be dried again to reduce the moisture to a level
(<0.1%) adequate for most applications. On the
basis of these physical properties, single-stage, cocur-
rent, and direct heat transfer rotary dryers and flash
dryers are commonly used. Rotary dryers have the
advantage of a longer residence time, making them
suitable for drying ABS polymers with a larger par-
ticle size. The flash drying system is suitable only for
small particle sizes but is more economical with re-
gard to thermal efficiency.

In case ABS forms lumps in the course of the

coagulation and/or dehydration process, it is neces-
sary to add another process to crush the lumps, i.e., to
install an FD with a cage mill or use a ring dryer in
the first stage of the dryer. Since drying in the falling
rate has the main objective of removing the mono-
mers, it is necessary for the material to have a long
retention time. To satisfy such a requirement, a batch
FBD is widely adopted.

Drying of ABS has been commercially successful

in a two-stage drying system with the combination of
direct and indirect heat transfer. Since ABS requires
both surface and bound moisture removal, a two-
stage drying system is recommended.

The two-stage flash FBD system is advantageous

in terms of thermal efficiency and product quality.
The first-stage flash dryer does most of the evapor-
ation. FBD, characterized by longer residence times,
is used in the second stage. In the second stage, FBD
can be replaced by a direct or indirect rotary dryer. If
a fluid bed is used as the second stage, it is advanta-
geous to use the plug-flow model since in such a bed
residence time can be controlled within narrow limits.

Among the developments for drying ABS are

the indirect-heated closed-loop, inert gas-heated, or
liquid-heated dryers. These dryers minimize the emis-
sion of styrene monomer and oxidation of the poly-
mer is prevented by the inert purge gas. The overall
efficiency is also high. A particular type of this class
of dryers is the indirect-heated FBD depicted in Fig-
ure 41.10. This type of dryer uses a rectangular bed to
optimize the solids flow and heat transfer fluid
LMTD effect. Also, the plenum-side inlet gas is at a
low temperature, precluding any mechanical con-
straints. In this process, an external direct-fired hater
operating at low excess combustion air heats a heat
transfer fluid (e.g., molten salt, thermal fluids, steam,
and others) to a temperature above that of the bed,
but below the ABS degradation temperature. Since
the heat source is decoupled from the fluidizing gas

Fired

heat

Fuel

Recycled N

blower

Fluide bed

Combustion air

Feed

Circulating fluid

Product

Scrubber

Stack

Cyclone

FIGURE 41.10 Contact closed-cycle fluidized bed dryer for acrylonitrile–butadiene–styrene.

2006 by Taylor & Francis Group, LLC.

source, large vessel diame ters are not ne eded. Fur -
ther, the smaller amou nt of fluidizi ng gas requir es
much smaller pollution con trol equipment .

W hen it is possible to obtain such wet raw mate r-

ial that is properl y coagulat ed and dehydrat ed but
with no form ation of lumps an d yet has a low level
of moisture, the singl e C-FBD as sho wn in

Figure

41.9

ha s been wi dely used in recent years.

ABS group resins are highly inflammable and self-

combustible and liable to cause dust explosion. It is
absolutely necessary to be very alert not only in set-
ting and controlling the hot air temperature but also
in eliminating any possible kindling causes, e.g.,
introduction of metallic foreign substances in the
raw material and overcharged static electricity. Care-
ful maintenance is further required. Periodical clean-
ing to remove the resin adhering to the equipment is
essential for safety.

ABS, while drying, emits styrene, a highly toxic

substance. Very recently the U.S. National Institute
for Occupational Safety and Health (NIOSH) has
set a limit for workplace exposure of styrene.
NIOSH suggests that workers should not be exposed
to >50 ppm of styrene over a time-weighted average
of 19 h/day, 40 h/week. Further, a ceiling concentra-
tion of 100 ppm during any 15-min sampling period is
enforced in the United States.

Owing to this recent regulation, there are indeed

very few optional routes left for drying ABS other
than indirect-heated drying with an inert closed-loop
gas system.

41.4.4 D

RYING OF

YNTHETIC

IBERS

Polymers that demand special precautions during
drying are common in the synthetic fiber industry.
Of these, nylon and polyester chips are the two most
common examples. These resins are hygroscopic and
have to be dried before a spinning or molding process.
Generally, these polymers are introduced to the dryer
in the form of 3- to 4-mm cubic pellets.

41.4.4.1 Nylon

Nylon is the generic term for any long-chain, syn-
thetic, polymeric amide in which recurring amide
groups are integral to the main polymer chain [3].
There is a wide choice of starting materials from
which polyamides can be synthesized. The two pri-
mary mechanisms for polymer manufacture are con-
densation of a diamine and a dibasic acid or their
equivalents or polymerization of monomeric sub-
stances. Nylons are identified by a simple numerical
system. The words polyamide and nylon are followed
by one or more numbers. One number indicates that

the product was prepared from a single monomeric
substance and also indicates the number of carbon
atoms in the linear chain of the recurring polymer
unit. For example, nylon-6 is manufactured by the
polymerization of caprolactam and nylon-11, from
11-aminoundecanoic acid. When two numbers are
used, they are separated by a comma and refer to
the reactants used in the polymer’s manufacture.
The first number refers to the number of carbon
atoms in the diabasic acid. Thus, nylon-6,6 is
prepared from the reaction of hexamethylenediamine
and adipic acid. The difference in numbers of carbon
atoms between the amide groups results in a signifi-
cant difference in mechanical and physical properties.
Although the theoretical number of nylon types is
very large, a few are commercially available. Of
these, nylon-6 and nylon-6,6 comprise about 75 to
80% of the nylon fiber and nylon-molding compound
market.

Nylon chips are normally dried form 4 to 10%

inlet moisture (wb) to <0.1% outlet moisture. If they
are allowed to absorb moisture, they must be dried
prior to processing. Some nylon may hold as much
as 2% moisture under normal storage conditions but
must still be processed satisfactorily with less than
0.1% moisture remaining in the material for reuse.
Because of the low temperature limits (70 to 808C)
allowable for drying nylon, very low dew points and
longer times are required to achieve even this much
dryness. The common dryer for nylon is the batch
vacuum tumble dryer. The drying temperature is
kept controlled within 70 to 808C, and drying time
ranges from 10 to 24 h. If vacuum drying is not
possible, use of recirculating dyers at 808C and
dehumidified air is the next best solution. During
hot, humid weather, attention must be paid to
guarantee that the recirculating air is indeed dry
or moisture will be added to nylon rather than
removed. Prolonged exposure to this drying condi-
tion can result in discoloration and possible prop-
erty deterioration.

Nylon has a poor polymerization effect, and the

chips have a high moisture content at the beginning
with a propensity for holding rather low levels of
moisture very tenaciously. As a result, a long time is
required for drying. For these reasons, it is advanta-
geous to use FBD and/or PDD for this process. In
fact these dryers can perform drying down to 0.002%
moisture content in 4 to 6 h.

Another characteristic of the nylon is that, if it is

at low moisture content, it is subjected to oxidative
deterioration and discoloration at high temperatures.
Because of this problem, it is usual to dry it with air
when the moisture content is high and then to dry in
an inert atmosphere.

2006 by Taylor & Francis Group, LLC.

41.4.4.2 Polyester

A polyester fiber is any long-chain synthetic polymer
composed of at least 85 wt% of an ester of a dihydric
alcohol (HOROH) and terephthalic acid (TA) ( p-
HOOCC

COOH). The most widely used polyester

fiber is made from linear polyethylene terephthalate
(PET).

PET is a linear homopolymer, i.e., a condensa-

tion polymer of TA or its dimethyl ester, dimethyl
terephthalate (DMT), and ethylene glycol. The poly-
mer is melted and extruded or spun through a
spinneret, forming filaments that are solidified by
cooling in a current of air. The spun fiber is drawn
by heating and stretching the filaments to several
times their original length to form a somewhat
oriented crystalline structure with desired physical
properties.

During early stages of processing of PET, drying

was carried out in batch vacuum tumblers. The pro-
cessing time was 10 to 12 h. As the demand for larger
capacity gradually increased, the multistage, batch-
type fluidized bed drying system replaced the older
vacuum tumbler dryers.

A characteristic of a PET chip is that, if the raw

material is heated at 90 to 1008C, its composition is
rearranged from a vitreous to a crystalline form. The
chips stick to each other owing to surface melting
when they are heated at a high temperature. In
order to avoid this problem, the drying system is
divided into two stages. In the first stage crystalliza-
tion and preheating are accomplished; in the second
stage drying is completed. In the first stage, the
heating is gradual. Agitation is required to prevent
sintering or sticking of the product at this stage.
Usually, a fluidized bed or agitated vessel is used for
this purpose.

After surface crystallization is performed, the

chips do not show adhesiveness before the tempera-
ture rises to the melting point. Advantage is taken of
this property of the chips, which are then discharged
into a continuously moving bed dryer. Usually nitro-
gen, with a dew point temperature of

408C, or de-

humidified air is passed countercurrent to the product
flow. In continuous operation, a 2-h gain in residence
time could be achieved.

In recent years, with the diversification of the

applications of PET, there is a demand to miniaturize
the equipment and to save energy. This has motivated
various special dryer designs exclusively for PET. One
is PDD. A combination of B-FBD and PDD has a
chip retention time close to that of an ideal piston
flow, thus enabling considerable savings in the energy
cost for drying.

41.4.5 M

ISCELLANEOUS

Polystyrene (PS) and acrylonitrile–styrene (AS) are
two other polymers produced in bulk quantities. Pre-
viously, these polymers were dried with FD. Later,
C-FBD replaced all previous FD dryers because of
their energy savings advantage. In recent years, pad-
dle dryers have made rapid gains. The heat-resisting
power of these materials is comparatively low. Melted
material will adhere to the walls of the equipment if
the processing temperature is not properly regulated.

PC is another commodity resin that demands

careful drying. When the polymer was first commer-
cialized, it was common to use FD plus B-FBD with
the steam-stripping process. In recent years this has
been gradually switched to PD with the idea of energy
saving and of the direct process of chloride solvent
without steam stripping. The Solidaire dryer is an-
other possible choice. Since PC has comparatively
high heat resistance, the drying process is not difficult.

Polypropylene oxide (PPO) is a recently developed

resin with an application that is rapidly expanding. It
requires a comparatively long drying time since it
contains superfine particles and has high affinity for
water. Of various kinds of polymers, this is the one
that requires the most difficult processing techniques.
The paddle dryer is found to process this material
economically.

41.5 DRYING OF POLYMER RESINS

In order to avoid surface defects in molded parts and
sheets made from resins, it is usually necessary to dry
the pellets before processing. Residual moisture above
some critical level can cause a finished product with
unsatisfactory surface finish and properties. Drying is
required to reduce the moisture content of the pellet
below some critical value. The degree of dryness de-
pends on the specific nature of each converting oper-
ation; some require more critical moisture control than
others. For example, PET and nylons are very hygro-
scopic but for different reasons. PET in normal storage
conditions contains about 0.15% moisture (db). It must
be dried to a level of 0.005% (db) or better for process-
ing. Although PET is not difficult to dry because of the
high temperature that can be used, it can have abso-
lutely no exposure to atmosphere between drying and
processing operations. On the other hand, some nylons
may hold 2% moisture under normal storage conditions
but can be processed satisfactorily with 0.1 to 0.15%
moisture in the material. Because of the low tempera-
ture limits (70 to 808C) allowable when drying nylon,
very low dew points and longer drying times are re-
quired to achieve even this much dryness.

2006 by Taylor & Francis Group, LLC.

41.5.1 G

ENERAL

BSERVATIONS

Depending on the degree of affinity for moisture, plas-
tic resin s can be divide d into tw o c lasses: (1) hy gro-
scopic and (2) nonhy groscopi c. Moistu re ad sorption
and/or absorpt ion capab ility de pends on the type of
resins as well as the ambie nt tempe ratur e in whi ch it is
placed. In some insta nces, expo sure of only few min-
utes can be detriment al. If the material is expo sed to a
certain tempe ratur e and relative humidi ty for a period
of time, it wi ll reach the equilibrium point, referred to
as the equilibr ium moi sture conten t (EMC ). Prior to
drying it is impor tant to know the permea bility (prod-
uct of the diffusion constant of water vapo r–polym er
system and the solubi lity coeffici ent) of polyme r to
water vap or since this dictates the conditio n for rela-
tive humidi ty for the safe storage of the polyme r [16].

41.5.1 .1 Nonh ygroscopi c Re sins

Polyethy lene, polystyrene, and PP fall unde r the clas-
sification of nonhy droscopic resins. These types of
polyme r resins collec t mois ture on the surfa ce of the
pellet only. The moisture can or iginate from severa l
potenti al sources . Such mois ture in some cases can be
remove d very easil y by moderat e preh eating imm edi-
ately prior to feedin g the mate rial into the mold. In
some cases it is suffici ent to provide vents at the
transiti on from the hopper to the mold cavity. In
some sit uations the mois ture can be remove d by pass-
ing war m air ov er the mate rial. The equipment util -
ized to heat air and dry resi ns is usually very sim ple,
e.g., an inlet air filter, a blow er, and a controlled
electric heater, as sho wn in Figure 41.11.

41.5.1 .2 Hygr oscopic Resins

PET, ny lon, ABS , and PC come unde r the class ifica-
tion of hygro scopic resi ns. Thes e types of polyme r
resin collect mois ture inside the pellet its elf. Remo val
of this moisture requir es dry air as well as he at. These
resins therefo re demand prop er design and carefu l
machi ne selection for each ap plication. Desiccant
dryers are the dominant techn ology for these resi ns.

41.5.2 D

RYING

ETHODS

41.5.2 .1 Dry ing with Heat as Transfer Med ium

In these types of dryer, air is us ed exclus ivel y as a he at
transfer medium . A dist inction is made be tween

1. Dryers with fresh air only (open syst em)
2. Air circul ation dryers with pa rtial supply of

fresh air (sem iopen system)

3. Desiccant dryers

Figu re 41.11 shows the simplest type of dryer wi th

fresh air operati on. Heated air flows through the bed
of granu les, nor mally from bottom to top, unifor mly
heat the bed of granule s and at the same time carry off
the mois ture. The a ir tempe ratur e at the inlet is kep t
approxim ately 20 8 C ab ove the de sired tempe ratur e of
the gran ule bed . Its advan tages are: (1) these units a re
inexpensi ve; (2) easy to handle a nd clean; (3) readily
attachable to molding machines; and (4) have a high
degree of efficiency (30 to 80%). Among its disadvan-
tages are: (1) drying depends on dew point temperature
(i.e., weather and climate); (2) it has only a moderate
efficiency for hygroscopic resins (20 to 30%); (3) there
is possible contamination of environment and gran-
ules (pollution); and (4) the exhaust air is at a high
temperature (40 to 608C). Usually these types of hop-
pers or dryers are suitable for nonhygroscopic plastics,
e.g., polyolefins and polystyrene.

In dryers with partial recir culation (

Figur e 41.12

) ,

all the exhaust is not vented into the atmosphere since
it still contains energy. Instead, 70 to 90% of the
exhaust air is recirculated. The fresh air makeup usu-
ally ranges from 10 to 30% of the total flow, which
increases the drying capacity of the recirculated air.
These dryers are more energy efficient than the open
type but have the same relative advantages and dis-
advantages and are particularly suitable for nonhy-
groscopic and mildly hygroscopic resins, e.g., ABS,
PC, PMMA, PPO, and SAN.

In desiccant dryers, the heat transfer medium

(generally air) is given an additional treatment to
lower the dew point in a desiccant chamber, where
the moisture is removed. This dried air passes through
a heating chamber and the fixed bed of granules from

Waste air filter

Hopper

Heater blower

Fresh air
filter

FIGURE 41.11 Resin dryer (open cycle).

2006 by Taylor & Francis Group, LLC.

bottom to top. Two most commonly used desiccants
are silica gel and molecular sieve. New polymeric
desiccants have recently been developed. When the
inlet concentration of moisture in the airstream is
high, silica gel removes more moisture by weight.
For lower inlet moisture conditions, the molecular
sieve works best. The incoming airstream to the des-
iccant bed in a plastic dryer is warm, generally above
408C. This makes use of a molecular sieve necessary
to remove moisture. Another advantage of the mo-
lecular sieve is that it produces 1000 kcal/kg of mois-
ture absorbed. As a result, a bed with a molecular

sieve is not only capable of achieving lower moisture
dew points, but also requires less energy input (as
heat) to achieve drying rates.

Figure 41.13 shows a desiccant bed system in the

semiopen design. In this unit the smaller stream,
heated to approximately 2008C, passes through a
desiccant chamber where moisture is removed from
the adsorbent and is then vented. After this regener-
ation of the adsorbent, the chambers are rotated.
Commercial units are also available in which the
circulating air is not exchanged. This design features
redrying in one chamber at a time by preheating fresh
air. Since in such units the entire desiccant battery is
removable, the adsorbent is redried outside the gran-
ule-drying circuit. This ensures almost constant dry-
ing capacity. By cooling the returning airflow with an
additional cooler, it is possible to lower the dew point
far below the ambient temperature.

Generally,

hygroscopic

resins,

e.g.,

nylon-6,

nylon-6,6, PET, PBT, and ABS, are dried in desiccant
dryers. The dew point of the drying medium has a
significant impact on drying hygroscopic resins. For
example, PET absolutely requires dew points in the

40 to

508C range to be adequately dried. For other

hygroscopic resins, dew points in the range of

15 to

258C are adequate.

41.5.2.2 Drying without a Heat Transfer Medium

An alternative to drying polymer resins is the use
of vented barrels for drying without a transfer med-
ium. This technology for drying resins is gaining
ground. In one of the proprietory designs, an annular

Waste air filter

Fresh air

Blower

Heater

Hopper

FIGURE 41.12 Resin dryer (semiopen cycle).

Feed

Fresh air

Air heaters
(reversible)

Desiccant batteries

Control valve (reversible)

Air heater

FIGURE 41.13 Resin dryer with desiccant batteries.

2006 by Taylor & Francis Group, LLC.

chamber forme d by a tubing made of mesh in the
center of the dryer an d a perfor ated exter nal shell or
barrel surroun ding the flowin g mate rials are used .
The mate rial enters the feed port on the top of the
dryer and then flows by gravi ty. The perfor ated shell
is covered with bands to heat the mate rial, an d the air
is drawn through it. This assem bly is furt her enclosed
in an exter nal protective jacket . As the mate rial flows
through the ann ular chamber, air en tering between
the protect ive jacket and the inner perforated barrel is
heated and draw n up the mesh tub ing or ch imney in
the center of the dryer. Air travel is control led by a
compres sed-air venturi. As he ated air passes through
the plastic granu les, it drives off su rface mois ture and
preheat s the mate rial before it enters the feed throat
of the screw. Any inter nal mois ture remai ning in the
hot pelle ts is flashed off almos t inst antaneous ly by
shearin g acti on. Water vap or flashed off in the barrel
is dr awn out by the mesh tubing ch imney, pro viding
an unobstruct ed escape path for the mois ture, which
is exhau sted into the atmos phere.

Advan tages of ve nting are: (1) littl e risk of co n-

taminati on; (2) ope ration indep endent of mois ture
content ; (3) reliabil ity; (4) consis tency of qua lity;
and (5) remova l of resi dual mon omers unde r favor-
able co ndition s.

41.6 DRYING OF SELECTED POLYMERS

From the discus sion above, it is obv ious that there is
an application for severa l dryer types for drying of
polyme rs and resi ns. For instan ce, suspensi on PVC is
usually dried eithe r in two-st age systems involv ing a
flash predryer followe d by a fluid -bed second-s tage
dryer (w ith or wi thout tubes) , or by a singl e drying
stage such as a rotar y drum dryer or , more com-
monly, an FBD wi th inter nal he at trans fer surfa ces
(e.g., tubes, coils, or plate s). Em ulsion PVCs, on the
other ha nd, are mainly process ed in spray dryers. PP
is dr ied in similar syst ems to those used for suspe n-
sion PVC , and the various form s of polyst yrene a re
process ed in eithe r flash or FBD s. Polyacry lonitril e,
which is frequent ly produced as a filter cak e, is dr ied
either on a ba nd dryer after being preformed into a
suitable shape, or dried in a single- or two-st age flash
dryer. Some form s of polyet hylene requir e drying and
here again systems are either flash dryers or FBD s
with in-bed heat trans fer tubes. For polyam ides, col-
umn dryers are mainly used unde r a nitrog en blan ket
in order to avo id oxidat ion. Some app lications em-
ploy low-temp erature fluid beds to dry the granule s.

Polyester granules (e.g., PET) are used for the pro-

duction of bottle polymer, film (either video or wrap-
ping), and fiber or filament. These types of polyester
require quite a different system as the product first

undergoes a crystallization stage before reducing the
moisture to very low levels, below 50 ppm. Due to the
very special requirements for this type of polymer, spe-
cial processing systems have been developed. The fol-
lowing presents an application for the economic drying
of polyester chips to very low moisture for the produc-
tion of microfilaments. The same drying systems can be
used for any of the other polyester products, as well as
for the drying of PBT and some PC granules.

In recent years, the trend in the produ ction of

polyester yarn is to produce ultrafin e micro filamen t
at 5 to 7 mm diame ter that requir es drying to 20 ppm .
Tradit ional filamen t yarn and staple fibe r having a
diame ter of 18 to 22 m m typic ally requir e 50-ppm
final moisture.

The first continuous PET drying syst em, origin -

ally develope d by Rosin Engi neering (Londo n), was a
combinat ion of horizont al pa ddle crystall izer with a
vertical column dryer. This syst em was used exten-
sively for the prod uction of all types of fibers, e.g. ,
indust rial yarn, bottl e polyme r, and film. Alt hough
quite versat ile in that it can be used wi th diff erent
types of granules having completely different sizes, it
has the disadvantage that there is a slight formation
of dust due to the mechanical action of the paddles,
and also that space has to be left at one end of the
crystallizer for the withdrawal of the rotor shaft.
However, at the same time, fluidized bed units for
solid-phase polymerization (SPP) of PET and poly-
amide were being developed. It was observed that
there were several advantages in using a fluidized
bed for the initial heating and precrystallizing phase
as compared with the rotary paddle type of other
existing systems. Rosin manufactured its first com-
bined fluidized bed crystallizer and column dryer for
PET drying in 1970 (

Figur e 41.14

) .

The system consists of a fluidized bed heater/pre-

crystallizer and a column dryer for PET. The fluid-
bed section has five main functions:

1. Evaporation of surface and some internal

moisture from PET

2. Transformation of PET from the amorphous

to the crystalline condition

3. Heating of the chips to the temperature re-

quired for drying in the column

4. Provide sufficient turbulence to avoid sintering

or chips sticking together

5. Removal of dust from the incoming granules

PET chips are fed into the fluid bed by a variable-

speed vibro feeder and a fixed-speed rotary valve that
acts as a gas seal. They meet an oncoming stream of
heated gas (e.g., nitrogen or air) and become partially
suspended in the flow. As more chips are fed in, the

2006 by Taylor & Francis Group, LLC.

fluid bed becomes established as a deep agitated mass
of material exhibiting many properties of a fluid.

When a wet chip (typically 0.5% moisture) falls

into the fluid bed, the surface moisture is rapidly
evaporated. As the chip is then further heated, crys-
tallization

occurs.

This

amorphous-to-crystalline

transformation of PET is an exothermic reaction
and the heat given off is quite sufficient to raise the
surface temperature of PET to above the softening
point. If the chips are not moving as they do in the
fully developed fluidized state, this will produce large
solid lumps of agglomerated chips.

It is important to prevent agglomeration of chips

so that the subsequent drying stage may proceed
uniformly. Agglomerates may not dry completely,
which gives rise to subsequent processing problems,
particularly with microfilament production. In add-
ition, agglomerates can lead to material flow prob-
lems if left unchecked and can shut down entire
operations. Plug flow in the fluid bed is achieved
through a system of internal baffles (which can be
adjusted if necessary to alter the residence time) so
that each chip is given a very similar thermal treat-
ment and therefore achieves uniform crystallization.

Bag filter

cyclone

Product
inlet

Vibro

feeder

Shut-off

valve

Fines bin

Secondary

filter

Fluid-bed

crystallizer

Weir

Bleed off

Heater

Damper

Fluid-bed fan

Column

dryer

Dehumidifier

Column air

heater

Vibro discharge

Rotary valve

Air or N

Rotary valve

–40

⬚C

Dewpoint

FIGURE 41.14 Continuous fluid-bed crystallizer/column dryer for polyester. (Courtesy of Rosin Engineering/Rosin
Americas Ltd.)

2006 by Taylor & Francis Group, LLC.

W hen the c hips reach the last section of the fluid

bed in the cryst allized a nd he ated cond ition, they pa ss
over a weir an d descend into the column dryer. Thi s
unit was origi nally developed by Rosin in con junction
with ICI over a pe riod of 2 y to arrive at a design that
gives true piston or plug flow.

A constant , gentl e flow of heated, dehumidi fied air

or nitro gen is pr ovided upwar d through the column.
The de w poin t of this gas is caref ully co ntrolled by
either a molec ular sieve ab sorption system when us ing
compres sed air, or a combinat ion of two -stage re-
frigerat ion/drying system for low-pres sure gas sup -
plied by a fan. The compres sed air syst ems are
more cost effective for plants wi th capacit ies up to
1000 kg /h. Above this rate, the ambien t air system
is usuall y preferred. When nitr ogen is involv ed, it
becomes more cost effecti ve to us e a closed-loop ,
low-pres sure de humidifyi ng system for all but the
smallest of plant cap acities.

The pro duction of ch ips with a fina l mois ture

content of 50 ppm typic ally require s a resid ence time
of 2 h using heated gas with a de w point of 40 8 C.
Whereas this mois ture level is sati sfactory for normal
yarns and staple fibers, micr ofilame nt quality requir es
a mois ture level of 20 ppm , which is achieve d by
corres ponding adjustment s to the retention tim e and
gas de w point.

Figure 41.14

, the gas supply to the column

dryer is heated after de humidifi cation using a simila r
heat exchanger as in the fluid bed. This relative ly
small amou nt of gas mixes wi th the gas above the
fluid bed and the same quan tity is then vented off to
atmosp here (air) or to a return line (nitrogen) afte r
the supply fan, such that the whol e syst em is ke pt in
pressur e balance. This small air loss is in fact the only
energy loss in the syst em. Sin ce the gas for the fluid
bed is recycled , the heat inpu t closely matche s that
requir ed to he at the chips, whi ch in any case are
heated for the subsequ ent extrusion pr ocess. In
many inst ances, the column dryer is posit ioned dir-
ectly above the extrude r.

41.7 CONCLUSION

It is clear from the discus sion in this ch apter that
numerous dr yer types can be used for drying of poly-
mers. Rotating doubl e-cone vacuum dryers, e.g., can
be used up to 300 8C and 0.1 torr a bsolute pressure for
process ing PET, PBT, and liquid-cry stal polyme rs.
The combinat ion of high tempe rature and low pres-
sure assists semicryst alline an d nonc rystalline poly-
mers to cryst allize and align to increa se the streng th
of the polyme r. Polyest er, nylon, fluorop lastics, and
polyuret hane can also be dried in rotating- cone

vacuum dryers . Suc h dry ers can hand le flakes, chips,
pellets, and crystals . The slow tumbl ing actio n doe s
not cause changes in the shapes of particles . Units a re
commer cially available in v olume from 0.2 to 350 ft

For solvent -wet mate rials, closed- syst em ope ration
includin g solvent recove ry is possibl e. Highe r main-
tenance costs are the lim itations of such dryers .

Among the new types of dryers suited for poly-

mers, one may cite the centri fugal pellet dr yer mar-
keted by Gala Industr ies (Eagle Rock, VA), which
can be used to dry polyet hylene, PP, polyester, rub -
ber, and so on in three distinct phases: (1) predew a-
tering; (2) impac t dewateri ng; and (3) air drying. Up
to 95% of the water is remove d by impac t and gravi ty
through vertical perfor ated plates in the first stage .
The prede watered pellets are fed into a turbin elike
rotor encased in stationar y cy lindrical screens . As
the pellets move spira lly (in the second stage ) from
the bottom to the top of the rotor, the wat er co ntent
is reduced to a value between 0.5 and 1%. Finally , air
drying reduces the mois ture to below 0.05% in the
upper part of the rotor as air is forced through the
moving pellets.

Numer ous pa pers ha ve appeared in recent years

on drying of polyme rs and resi ns. Shah and Aroara
[17] have reviewed the stat e of the art of drying sus-
pension- PVC. They compare, in depth, continuous
FBDs, rotary dryers , and cyclone dryers. The impac t
on en ergy consumpt ion, maint enance co sts, and
product qualit y is assessed an d compared. They
show that FBD wi th immersed heat exch anger ha s
some limitations when severa l grades of polyme rs a re
to be dried with frequent g rade ch anges.

Reader s inter ested in mathe mati cal mo deling of

polyme r drying may refer to Vergnau d [18] .

Fol lowing is an exampl e of how selec tion of the

dryer is affected by qua lity of the dried product that
may be used as raw mate rial to prod uce differen t
consumer prod ucts. Sha h and Arora [19] ha ve sur-
veyed the various possible drye rs used for cryst alliza-
tion/dryi ng of polyest er chips from initial moisture
content of about 0.3 to 0.5% (wb) to under 50 ppm .
Aside from low average moisture con tent it is also
necessa ry to ensure uniform distribut ion of mois ture,
especi ally for certa in prod ucts, e.g. , producti on of
thin films . The unifor mity con straint is less severe if
the chips are to be used to make PET bottles.

Figure

41.15

shows schema tics of the cryst allizat ion/drying

steps involv ed. Genera lly, it is a two -step process . The
material is heat- sensi tive. The init ial cryst allizat ion/
drying is fast er than the drying step at low mois ture
levels. A two-stage dryer is indicated and is com-
monly used. It is possible to use different dryer types
for each stage as shown in

Figure 41.16

. A single

dryer type (e.g., column or packed bed dryer with

2006 by Taylor & Francis Group, LLC.

the chips moving downw ard slowly unde r gravity) is
cheaper a nd hen ce recomm ended for the lowe r qual-
ity grade but a more exp ensive fluid bed followe d by
another fluid bed or co lumn dryer may be ne eded for
the higher quality g rade. Note that numerou s a lter-
natives are possibl e in each case. It is also impor tant
to ope rate the dryers at the correct con ditions of gas
flow rate, tempe ratur e, and hum idity. Dehu midified
air is needed to achieve low final moisture content s in
accordance with the equili brium moisture isot herms
of the prod uct.

Ano ther example of dryer selec tion is related to

the choice of a suitab le atomizer for a spray dryer.
A spray dryer is indica ted when a pumpab le slurr y,
solution , or suspensi on is to be reduced to a free-

flowin g powder . With pr oper choice of atomi zer,
spray chamber de sign, gas tempe ratur e, and flow
rate it is possibl e to ‘‘eng ineer’’ powder s of desired
particle size and size dist ribution.

Tabl e 41.2

shows

how the ch oice of the atomizer affe cts chamber
design, size, as well as energy con sumption of atom-
ization and particle size dist ribution . The newly devel-
oped tw o-fluid sonic nozzles appear to be especi ally
attractiv e choices when ne arly mon odisper se powder s
need to be produ ced from relative ly moderat e viscos -
ity feeds (e.g., under 2 50 cp) at capacit ies up to 80 t/h
by using mult iple nozzles. M ore exampl es may be
found in Kudra and Mujumdar [21].

New drye rs are be ing developed con tinuous ly as a

result of industrial demand s. Over 2 50 U.S. patent s

Column dryer with internal tube

Multistage fluid bed

Wet chips

500–1000 ppm

Moisture

<50 ppm

Moisture

Crystallizer/

dryer

Final Dryer

Continuous

Direct

Continuous

Direct

Batch

Indirect

Batch

Indirect

Vacuum tumbler

Fluid bed

Vibro-fluid bed

Pulsed fluid bed

Vortex (spouted) bed

Column dryer (with mixer)

Paddle dryer

Vacuum tumbler

Paddle dryer

FIGURE 41.15 Schematic diagram of crystallization/drying steps in the production of polyester chips.

e.g., for magnetic tape e.g., for speciality fiber,

film

e.g., for PET bottles,
staple fiber, etc.

Polymers chips:

quality parameter

High

Medium

Average

A. Crystallizer: fluid bed

B. Finish dryer:
multistage fluid bed
with dehumidified air

A. Crystallizer: fluid bed
or pulse fluid bed or
paddle crystallizer

B. Finish dryer:
column dryer with a
central tube for
smooth downward
flow of chips

Single column
crystallizer/dryer with
a mixer in the top
crystallizer section
to avoid agglomeration

Low capital/operating
cost, smaller space
requirements

FIGURE 41.16 Possible dryer types for drying of polyester chips.

2006 by Taylor & Francis Group, LLC.

are grante d each year related to dryers (equipm ent)
and drying (proces s); in the Eur opean Com munity
about 8 0 patent s are issue d a nnually on dryers .
Kudra and Mujumdar [21] have discus sed a wide
assortment of novel drying techn ologies, whi ch are
beyond the scope of this chapter . Suffice it to note
that many of the ne w technol ogies (e.g., superhea ted
steam, pulse comb ustion— new gas-pa rticle co ntac-
tors as dryers ) will eventual ly rep lace conv entiona l
dryers in the next de cade or two. Among the mo re
popular new dryers for polyme rs is the pulsed bed
dryer. This dryer uses pulsat ing motion imparted to
the bed of parti cles by periodicall y reloca ting the
fluidized region of the vessel. This type of dryer ha s
been claimed to ha ve a higher e fficiency and lower air
consumpt ion for fluidiza tion and for dry ing. It is
discus sed in detai l by Kudra and Mujumdar [21] .
New techno logies are inherent ly mo re risky and
more difficult to scale -up. Hen ce, there is natural
reluctan ce to their adop tion. Reader s are encouraged
to revie w the ne w de velopm ents in order to be su re
that their selec tion is the most appro priate one for the
applic ation at hand.

It is well known that most polyme rs leaving the

polyme rization react or con tain various but smal l
amounts of unreact ed monomer , solvents, wat er,
and/or va rious react ion by -product s. The presence
of these volat iles in the polyme r is undesir able. Thei r
concen trations may range from severa l pa rts per mil -
lion to severa l tens of percen tage. Their separation
from bulk polyme r is necessa ry to impr ove pol ymer
propert ies, to recover monomer an d solvent s, to meet
health an d e nvironm ental regula tions, to elim inate
odors, and /or to increa se the extent of polyme riza-
tion. This pro cess of devo latilizati on is us ually per-
formed abo ve the glass transition tempe ratur e of the
polyme r. The read er is refer red to Alba lak [20] for

detailed discussion of the theory of devolatilization
and various devolatilizing equipments.

ACKNOWLEDGMENTS

The authors are grateful to S.N. Rosin (Rosin
Engineering, London) and Michael Spino (Rosin
Americas, Montreal) for the contents of Section 6 of
this chapter and

Figure 41.14

. We are grateful to

Purnima and Anita Mujumdar for their assistance in
preparing this chapter.

REFERENCES

1. Oringer, K., CEP, 68(3):96–190 (1972).
2. Mujumdar, A.S., Ind. Inst. Chem. Engrs., 4:98–106

(December 1981).

3. Driver, W.E., Plastics Chemistry and Technology, Van

Nostrand, New York, 1979.

4. Lenz, R.W., Organic Chemistry of Synthetic High

Polymers, Interscience, New York, 1967.

5. Dittman, F.W., Chem. Eng. NY, 84(2):106–108 (1977).
6. Mujumdar, A.S., Industrial drying systems seminar,

Paper No. SN-4, McNeill & Magor, Bombay, India,
1984.

7. Funaoka, R., Industrial drying systems seminar, Paper

No. SN-9, McNeill & Magor, Bombay, India, 1984.

8. Forthuber, D., CEP, 79(4):71–76 (1983).
9. Hass, D. and Rossi, R.A., CEP, 70(4):43–50 (1983).

10. Bepex Corporation, CEP, 79(4):5 (1983).
11. Yamato, Y., U.S. Patent 3,815,255 (1974).
12. Mujumdar, A.S., Industrial drying systems seminar,

Paper No. SN-12, McNeill & Magor, Bombay, India,
1984.

13. Glanvill, A.B., Plastics Engineering Data Book, Indus-

trial Press, New York, 1974.

14. Herron, D. and Hammel, D., CEP, 76(1):44–52 (1980).
15. Eberspacher, R., Plastics Eng., 36(7):25–28 (1980).

TABLE 41.2
Spray Drying of Emulsion-PVC. Effect of Selection of Atomizer on Spray Dryer Performance: A Comparison
between Different Atomizers

Parameter

Rotary Disk

Two-Fluid (Sonic)

Two-Fluid (Standard)

Dryer geometry

Conical/cylindrical
H/D

1.2–1.5

Tall-form cylindrical
H/D

Tall-form
Cylindrical H/D

Evaporation capacity (water) (kg/h)

1600

Chamber (D

H) (m)

6.5

3.5

Number of nozzles

1,175-mm disk

16 nozzles

18 nozzles

15,000 rpm

4 bar pressure

Power for atomizer (W/kg slurry)

Capital cost

High

Medium

Operating cost

Medium

Low

High

2006 by Taylor & Francis Group, LLC.

16. Roff, W.J. and Scott, J.R., Handbook of Common

Polymers, Butterworths, London, 1971.

17. Shah, R.M. and Aroara, P.K., in Drying ’92, Part B,

Mujumdar, A.S. (Ed.), Elsevier, Amsterdam, The Neth-
erlands, pp. 1311–1320 (1992).

18. Vergnaud, J.M., Drying of Polymeric and Solid

Materials, Springer, Berlin (1991).

19. Shah, R.M. and Arora, P.K., in Drying ’96, Strumillo, C.,

Pakowski, Z., and Mujumdar, A.S. (Ed.), Lodz, Poland,
pp. 1361–1366 (1966).

20. Albalak, R.J. (Ed.), Polymer Devolatilization, Marcel

Dekker, New York, pp. 722 (1996).

21. Kudra, T. and Mujumdar A.S., Advanced Drying Tech-

nologies, Marcel Dekker, New York, pp. 457 (2001).

2006 by Taylor & Francis Group, LLC.

Document Outline

Table of Contents
Chapter 041: Drying of Polymers

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