Handbook of Industrial Drying

DRYING OF SELECTED POLYMERS

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-

From the discus sion above, it is obv ious that theraemiisde were being developed. It was observed that
an application for severa l dryer types for drying tohfere were several advantages in using a fluidized polyme rs and resi ns. For instan ce, suspensi on PVCbedisfor the initial heating and precrystallizing phase usually dried eithe r in two-st age systems involv ingas acompared with the rotary paddle type of other flash predryer followe d by a fluid -bed second-s taegxeisting systems. Rosin manufactured its first com- dryer (w ith or wi thout tubes) , or by a singl e dbriyniendg fluidized bed crystallizer and column dryer for
stage such as a rotar y drum dryer or , more cPoEmT- drying in 1970 (Figur e 41.14) .
monly, an FBD wi th inter nal he at trans fer surfa ceTshe system consists of a fluidized bed heater/pre-
(e.g., tubes, coils, or plate s). Em ulsion PVCs, on ctrhyestallizer and a column dryer for PET. The fluid- other ha nd, are mainly process ed in spray dryers. bPePd section has five main functions:
is dr ied in similar syst ems to those used for suspe n-
sion PVC , and the various form s of polyst yrene a r1e. Evaporation of surface and some internal
process ed in eithe r flash or FBD s. Polyacry lonitril e, moisture from PET
which is frequent ly produced as a filter cak e, is dr ie2d. Transformation of PET from the amorphous
either on a ba nd dryer after being preformed into a to the crystalline condition
suitable shape, or dried in a single- or two-st age flash3. Heating of the chips to the temperature re-
dryer. Some form s of polyet hylene requir e drying and quired for drying in the column
here again systems are either flash dryers or FBD s 4. Provide sufficient turbulence to avoid sintering
with in-bed heat trans fer tubes. For polyam ides, col- or chips sticking together
umn dryers are mainly used unde r a nitrog en blan ke5t. Removal of dust from the incoming granules in order to avo id oxidat ion. Some app lications em-
ploy low-temp erature fluid beds to dry the granule s. PET chips are fed into the fluid bed by a variable-
Polyester granules (e.g., PET) are used for the prosp-eed vibro feeder and a fixed-speed rotary valve that
duction of bottle polymer, film (either video or wrapac-ts as a gas seal. They meet an oncoming stream of ping), and fiber or filament. These types of polyesthereated gas (e.g., nitrogen or air) and become partially require quite a different system as the product firssutspended in the flow. As more chips are fed in, the

Weir

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

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.

W hen the c hips reach the last section of the flvuaciduum dryers . Suc h dry ers can hand le flakes, chips, bed in the cryst allized a nd he ated cond ition, they ppeallestss, and crystals . The slow tumbl ing actio n doe s over a weir an d descend into the column dryer. Tnhoits cause changes in the shapes of particles . Units a re unit was origi nally developed by Rosin in con juncticoonmmer cially available in v olume from 0.2 to³.350 ft with ICI over a pe riod of 2 y to arrive at a designFotrhastolvent -wet mate rials, closed- syst em ope ration
gives true piston or plug flow. includin g solvent recove ry is possibl e. Highe r main-
A constant , gentl e flow of heated, dehumidi fied teanirance costs are the lim itations of such dryers .
or nitro gen is pr ovided upwar d through the column.Among the new types of dryers suited for poly- The de w poin t of this gas is caref ully co ntrollemd erbsy, one may cite the centri fugal pellet dr yer mar- either a molec ular sieve ab sorption system when us kinetged by Gala Industr ies (Eagle Rock, VA), which
compres sed air, or a combinat ion of two -stage craen- be used to dry polyet hylene, PP, polyester, rub -
frigerat ion/drying system for low-pres sure gas supb-er, and so on in three distinct phases: (1) predew a-
plied by a fan. The compres sed air syst ems taering; (2) impac t dewateri ng; and (3) air drying. Up
more cost effective for plants wi th capacit ies up toto95% of the water is remove d by impac t and gravi ty
1000 kg /h. Above this rate, the ambien t air systtehmrough vertical perfor ated plates in the first stage . is usuall y preferred. When nitr ogen is involv ed,Thite prede watered pellets are fed into a turbin elike becomes more cost effecti ve to us e a closed-loorpot,or encased in stationar y cy lindrical screens . As

low-pres sure de humidifyi ng system for all but smallest of plant cap acities.
the pellets move spira lly (in the second stage ) from the bottom to the top of the rotor, the wat er co ntent

The pro duction of ch ips with a fina l mois itsurreeduced to a value between 0.5 and 1%. Finally , air content of 50 ppm typic ally require s a resid ence tdirmyeing reduces the mois ture to below 0.05% in the
of 2 h using heated gas with a de w p oi4n0t8Co.f upper part of the rotor as air is forced through the
Whereas this mois ture level is sati sfactory for normmaol ving pellets.
yarns and staple fibers, micr ofilame nt quality requir es Numer ous pa pers ha ve appeared in recent years a mois ture level of 20 ppm , which is achieve donbydrying of polyme rs and resi ns. Shah and Aroara corres ponding adjustment s to the retention tim e a[n1d7] have reviewed the stat e of the art of drying sus-
gas de w point. pension- PVC. They compare, in depth, continuous
In Figure 41.14, the gas supply to the columFnBDs, rotary dryers , and cyclone dryers. The impac t
dryer is heated after de humidifi cation using a simiolanr en ergy consumpt ion, maint enance co sts, and
heat exchanger as in the fluid bed. This relativeplryoduct qualit y is assessed an d compared. They
small amou nt of gas mixes wi th the gas above shthoew that FBD wi th immersed heat exch anger ha s fluid bed and the same quan tity is then vented offsomtoe limitations when severa l grades of polyme rs a re atmosp here (air) or to a return line (nitrogen) aftoe rbe dried with frequent g rade ch anges.
the supply fan, such that the whol e syst em is ke pt Rineader s inter ested in mathe mati cal mo deling of pressur e balance. This small air loss is in fact the opnolyme r drying may refer to Vergnau d [18] .
energy loss in the syst em. Sin ce the gas for the fluiFdol lowing is an exampl e of how selec tion of the
bed is recycled , the heat inpu t closely matche s dthryaetr is affected by qua lity of the dried product that
requir ed to he at the chips, whi ch in any casemayre be used as raw mate rial to prod uce differen t
heated for the subsequ ent extrusion pr ocess. Icnonsumer prod ucts. Sha h and Arora [19] ha ve sur- many inst ances, the column dryer is posit ioned dvire-yed the various possible drye rs used for cryst alliza-

ectly above the extrude r.

4.