Handbook of Industrial Drying

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.

1. 
   GENERAL OBSERVATIONS
   

1. 
   .2 Hygr oscopic Resins
   

Depending on the degree of affinity for moisture, plaPs-ET, ny lon, ABS , and PC come unde r the class ifica- tic resin s can be divide d into tw o c lasses: (1) htyiognroo- f hygro scopic resi ns. Thes e types of polyme r scopic and (2) nonhy groscopi c. Moistu re ad sorptiroensin collect mois ture inside the pellet its elf. Remo val and/or absorpt ion capab ility de pends on the typeofofthis moisture requir es dry air as well as he at. These resins as well as the ambie nt tempe ratur e in whi chresiitnsis therefo re demand prop er design and carefu l placed. In some insta nces, expo sure of only few mmina-chi ne selection for each ap plication. Desiccant utes can be detriment al. If the material is expo sed dtroyears are the dominant techn ology for these resi ns. certain tempe ratur e and relative humidi ty for a period

of time, it wi ll reach the equilibrium point, referred⁴¹to^{.5.2 DRYING METHODS}

as the equilibr ium moi sture conte(nEtMC ). Prior to

2. 
   .1 Dry ing with Heat as Transfer Med ium
   

drying it is impor tant to know the permea bility (prod-
^{uct of the diffusion constant of water vapo r–polym}I^en^r these types of dryer, air is us ed exclus ivel y as a he at system and the solubi lity coeffici ent) of polyme r _trt_ao_{nsfer medium . A dist inction is made be tween}
water vap or since this dictates the conditio n for rela-
tive humidi ty for the safe storage of the polyme r [16]._{1. Dryers with fresh air only (open syst em)}

2. 
   Air circul ation dryers with pa rtial supply of
   

^{41.5.1 .1 Nonh ygroscopi c Re sins} fresh air (sem iopen system)

2. 
   Desiccant dryers
   

Polyethy lene, polystyrene, and PP fall unde r the clas-

sification of nonhy droscopic resins. These types of
Figu re 41.11 shows the simplest type of dryer wi th

polyme r resins collec t mois ture on the surfa ce of_fret_sh_he
air operati on. Heated air flows through the bed

pellet only. The moisture can or iginate from sever_oa_fl _{granu les, nor mally from bottom to top, unifor mly} potenti al sources . Such mois ture in some cases can_heb_ae_{t the bed of granule s and at the same time carry off} ^{remove d very easil y by moderat e preh eating imm} t^eh^de^i- mois ture. The a ir tempe ratur e at the inlet is kep t
^{ately prior to feedin g the mate rial into the mold.}ap^Ipⁿroxim ately 280C ab ove the de sired tempe ratur e of

some cases it is suffici ent to provide vents at
transiti on from the hopper to the mold cavity.
t^th^he^e gran ule bed . Its advan tages are: (1) these units a re
i^Inⁿexpensi ve; (2) easy to handle a nd clean; (3) readily

some sit uations the mois ture can be remove d by p_aa_ts_ts_a-_{chable to molding machines; and (4) have a high}

ing war m air ov er the mate rial. The equipment
d^ue^tig^lr^-ee of efficiency (30 to 80%). Among its disadvan-

^{ized to heat air and dry resi ns is usually very sim}ta^pg^le^es^, are: (1) drying depends on dew point temperature
^{e.g., an inlet air filter, a blow er, and a control}(^li^e.e^d., weather and climate); (2) it has only a moderate

electric heater, as sho wn in Figure 41.11.

FIGURE 41.11 Resin dryer (open cycle).

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

Hopper

Waste air filter

Fresh air

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

Heater
FIGURE 41.12 Resin dryer (semiopen cycle).
Blower
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

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
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

Fresh air

Feed
Air heaters (reversible)

Desiccant batteries

Control valve (reversible)

Air heater

FIGURE 41.13 Resin dryer with desiccant batteries.

chamber forme d by a tubing made of mesh in uthnedergoes a crystallization stage before reducing the
center of the dryer an d a perfor ated exter nal shemll ooisrture to very low levels, below 50 ppm. Due to the barrel surroun ding the flowin g mate rials are usevder.y special requirements for this type of polymer, spe- The mate rial enters the feed port on the top of citahle processing systems have been developed. The fol- dryer and then flows by gravi ty. The perfor ated shloewlling presents an application for the economic drying
is covered with bands to heat the mate rial, an d thoe f apirolyester chips to very low moisture for the produc- is drawn through it. This assem bly is furt her enclotsieodn of microfilaments. The same drying systems can be
in an exter nal protective jacket . As the mate rial fluosweds for any of the other polyester products, as well as
through the ann ular chamber, air en tering betweefnor the drying of PBT and some PC granules.
the protect ive jacket and the inner perforated barrel is In recent years, the trend in the produ ction of heated and draw n up the mesh tub ing or ch imnepyoliynester yarn is to produce ultrafin e micro filamen t
the center of the dryer. Air travel is control led bayt a5 to m7m diame ter that requir es drying to 20 ppm .
compres sed-air venturi. As he ated air passes througThradit ional filamen t yarn and staple fibe r having a

the plastic granu les, it drives off su rface mois ture dainadme ter of 18 to
preheat s the mate rial before it enters the feed thrfionaatl moisture.
m2m2
typic ally requir e 50-ppm

of the screw. Any inter nal mois ture remai ning in theThe first continuous PET drying syst em, origin -
hot pelle ts is flashed off almos t inst antaneous lyallbyy develope d by Rosin Engi neering (Londo n), was a shearin g acti on. Water vap or flashed off in the bacrormelbinat ion of horizont al pa ddle crystall izer with a is dr awn out by the mesh tubing ch imney, pro vidveinrtgical column dryer. This syst em was used exten- an unobstruct ed escape path for the mois ture, whiscihvely for the prod uction of all types of fibers, e.g. ,
is exhau sted into the atmos phere. indust rial yarn, bottl e polyme r, and film. Alt hough
Advan tages of ve nting are: (1) littl e risk of cqouinte- versat ile in that it can be used wi th diff erent
taminati on; (2) ope ration indep endent of mois tutyrepes of granules having completely different sizes, it content ; (3) reliabil ity; (4) consis tency of qua lhitays; the disadvantage that there is a slight formation and (5) remova l of resi dual mon omers unde r faovford-ust due to the mechanical action of the paddles,

able co ndition s.

4.