Handbook of Industrial Drying

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].

1. 
   DRYING OF POLYOLEFINS
   
   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	71
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	0.08	60–88

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

(0.532)
Dry basis


0.35
Flash

Post fluid-bed dryer, plug flow with gas

Dew point control

0.30

Fraction hexane. wet basis
0.25
0.20
0.15
DRT/backmix FB
Minimum fluidization homopolymer

0.10
0.05


0.02
0.0005
Minimum fluidization copolymer

Break point:

constant rate/falling rate

Low dew point drying gas

Product diluent

High dew point drying gas

Drying rate is mass transfer controlled
Drying rate is heat transfer controlled Time

FIGURE 41.6 Typical polymer-drying curve of polypropylene.

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].

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).

2. Drying of Polyvinyl Chloride

   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

FIGURE 41.7 Drying system for polypropylene and polyethylene.

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 is offered by 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.

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

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

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

FIGURE 41.9 Contact fluidizer for suspension-grade polyvinyl chloride.

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.

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.

3. Drying of Acrylonitrile– Butadiene–Styrene

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

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

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

source, large vessel diame ters are not ne eded. Futhre- product was prepared from a single monomeric ther, the smaller amou nt of fluidizi ng gas requirsuebsstance and also indicates the number of carbon
much smaller pollution con trol equipment . atoms in the linear chain of the recurring polymer
W hen it is possible to obtain such wet raw mautenirt-. For example, nylon-6 is manufactured by the
ial that is properl y coagulat ed and dehydrat ed pboultymerization of caprolactam and nylon-11, from
with no form ation of lumps an d yet has a low 1l1ev-aeml inoundecanoic acid. When two numbers are
of moisture, the singl e C-FBD as sho wn in Figuusreed, they are separated by a comma and refer to

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.

4. 
   DRYING OF SYNTHETIC FIBERS
   

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.

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 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.

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- HOOCC₆H₄COOH). 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.

5. Miscellaneous

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.

4.